Nice experiment, it is also the basis of which TDR (Time Domain Reflectometry) which is used to detect the distance to a break in optical fiber, or coaxial cable for RF.
Thanks!!! And yes TDR. I did a video essentially on that trying to give viewers some sense of reflections on transmission lines and why termination resistance is so important - if your interested, here it is: ua-cam.com/video/a-8CaGOmWDU/v-deo.html
The +-7% error is that primarily caused by your scopes or just where you choose to measure the peaks or blobby light and handheld mirror? If you used a focused laser and a mounted beam spliting prism would that give sharper peaks and remove a lot of the error?
I think it was more from where I choose to determine the peaks were. Noise in the measurement made it hard, and for the digital scope, when I re-look at the images, I probably read one of the peak positions a bit wrong. I think your right, a better focused beam would have provided more optical power to the detector diode and a mounted prism or mirror would have helped - me hand-holding the near mirror did not produce the most steady reflection. In fact, with a better mount, one could then average a number of scope traces to produce an almost noiseless result. With a bit of care I think this setup with the approximately 50' distance, getting a result within 2 or 3% is quite realistic.
Thanks and your welcome!! Yes - the results were good. I think with a low noise amp right at the photodiode once could reduce the noise levels on the scope and hopefully get significantly more accurate individual readings. But this was good enough!
It was very interesting, but what about the slow down on light velocity produced by the lents glass ? the short path doesn't pass at least one of the lents.
Your right about the short path missing a lens, so ideally we would put a lens somewhere in the second path so it equals the lenses in the long path. But - the reason I could "cheat" is we "know" that light going though glass slows down to perhaps half is speed in free space. The speed in airor free space is about 1 nano second per foot or 1 nano second for 30 cm. The lens is less that 1 cm thick at the thickest point, so in free air it would take 1/30 ns for do that distance, or in the less glass at half its normal speed, 2/30 ns. So removing the lens removes 1/30 ns from the round trip measurement, which is way less than the round trip time and way smaller than the accuracy we can obtain with this experimental setup. So removing one lens has no effect for a measurement/experiment that is as crude as this one. Now we really should prove this - that is measure how much light slows down in glass. Thats a future video! Thanks for the question!
I was quit pleased how simple it was to do in the end after some experimentation. Stable lantern - I didnt know it was a stable lantern. It is actually really old - when I was a kid a we lived in Johannesburg where my dad was posted for a few years. We found that lantern buried in an old compost heap in the garden of the house we were living in. The property had been a farm, so stable lantern makes complete sense. If you look closely you can see the writing on bottom part that says "City of Johannesburg".
Thanks a lot for the very thorough explanation of your setup. My son (4th grade) is learning about light in tech class this week, so I will definitely try and replicate your experiment at home!
So glad you liked it - would be fantastic if you can replicate it for your son - maybe even show his class? If you havnt seen it, I did a followup with more details about the parts I used and possible alternatives: ua-cam.com/video/37Kt1Hs1pOo/v-deo.html If you do it, please post how it worked and any changes you may have made to make it easier to do or better!
Thanks! Please do recreate it! It would be wonderful to hear your results when you do it and any improvements or simplifications of the experimental setup you may come up with. I hope the DIY video with be up in a week or so, time permitting....
Thank you so much! If you liked that, you might enjoy my wavelength of light video where I laser print a diffraction grating to do it: ua-cam.com/video/WwQXQrm33JM/v-deo.html
Perhaps you could make a calibration on the turn on delay of the laser diode and detector by putting them face to face as there may be a delays in these two parts.
Certainly - do that, note the photo detector delay (lower trace) from the upper trace (voltage to the laser) , and repeat with the return signal relative to the upper trace and take the difference. I'm detecting both outgoing and incoming light pulses on the same trace, so the difference in their peaks eliminates the effects of any laser turn on delays.
That Philips Scope looks exactly like the model I was using back in the day (1983) at GEC Coventry...setting up 11GHz QPSK modulators for the system x telephone systen in the UK
You nailed the time frame of that scope. Its a scope from the undergrad labs at the university I was a student at in the early 80s. Great scopes but the push buttons tended to wear out with daily lab use. 11GHz QPSK back then - wow - that must have been incredible at that time! Microwave links?
@@ElectromagneticVideos It was a time consuming task...With Microwave substrates and pushing around Gold 1 millimeter blocks to tune the Modulators in 0 - 90 - 180 - 270 degree phases. The Philips Scope was used to tune the modulators at the frequency range (around 11 GHz +/- 0.5 GHz ish) it traced out a datum straight line and you had a second trace which you had to get to follow the datum line, The setup must have used an equivelent, maybe DC voltage to indicate the Modulators response across the range and the scope displayed this. The frequency range spanned the width of the scope display. When you got it right, the two lines would be as close to one another as you could get. You then mixed an apoxy glue to fix the Gold blocks in place. To add to this, there were 5 magnets you could twist to help the tuning and they were glued in place with Silastic (This stank terrible).... When you thought you had it right, the Modulator would go into a small oven to cure the Silastic and epoxy. A couple of hours later you retrieved the Modulator and rechecked it....if the magnets or epoxy had moved and the scope trace was no longer following the datum trace within spec across the phases, then everything would have had to be redone, cleaning things before re-trying. Two a day was all you could hope for. I was just about 20 and didn't like it too much but it paid well.... 😁 Memory is a little fuzzy I think I remembered it ok. When I left GEC years later, 19GHz was coming in. While 2 GHz was old tech and the waveguides were about 2 inches wide internally, 11 GHz was about a third of that and 19 GHz was a lot smaller too.
Wow, what an insightful video! Watching it has inspired me to try out this technique myself. I've actually experimented with a similar method before, using ultrasonic transducers linked to a dual beam scope to measure the speed of sound. Aligning the source and received signals in phase on the dual channel scope and marking their positions on a ruler scale was quite an interesting process. Then, by adjusting the receiver transducer and noting the distance when the phase match occurred again, I could calculate the speed of sound. Additionally, I've utilized a comparable setup with a time-domain reflectometer (TDR) for measuring coaxial length or velocity factor using nanosecond pulses. The idea of applying this method to measure the speed of light never crossed my mind before. The historical context, especially the cog wheel method, struck me as particularly ingenious. Thank you for sharing such an informative and well-presented video!
So glad you liked it! If your going to do it, order one one of the laser diodes sooner rather than later - they seem to be in the process of being discontinued. Finding a diode with suitable specs (acceptable brightness, speed and cost) was not easy although there definitely others in production. Very cool you did the audio version! TDR - that was my favorite EM lab when I was a student. Actually did a video on it- disguised as "why doesnt a 75 Ohm cable measure 75 ohms".
Alright, I came to this video expecting it would be a waste of time, but you've honestly outdone yourself. This was a very interesting learning experience and although I'm sure we could agree that there may still be some potentially unknown variables in the setup we can definitely somewhat easily and convincingly measure a clear and reasonable result in this way.
Wow - thank you so much! Yes there are things that a purposely did not mention in the video to keep it a reasonable length and not clutter the core concepts. I'm working on a "part 2" with more details how to DIY and things that should be considered.
It just blocks it to make a beam that rapidly goes on and off. Same idea as what I did, except I used modern electronics to switch a laser on an off even faster so the experiment would be easy to do over a much shorter distance.
Fantastic! I had always assumed that it would be completely impossible to show a delay without hundreds of thousands of dollars in research grants and a fully equiped lab. Heck, I will be able to do this in my own basement with equipment I already have. Please do post that follow up video. I will be waiting, but I am sure I can work it out myself since you have shown it to be so simple. Thank you!
So glad you liked it! I'm working on the follow up video, and will include suggestions from viewers of this one. I would be thrilled if you do it and let me know the results and any improvements or simplifications to the experimental setup you might do. If you dont already have a photo diode and laser, I used PDB-C156 and PLT5 522EA_Q which I got from Digikey, but I think Mouser, Newark and other also have them. I di want to see if I can find a fast, bright LED to use in place of the laser to make things a bit safer.
@@ElectromagneticVideos Thanks, I do need the laser diode. One other question. I do need a real signal generator as well (but they are not expensive and I wanted one anyway). The JDS6600 comes in a number of versions up to 60 Mhs. Which model are you using? It looks like the 30M would be needed to get down to a 100ns pulse according to the specs, but that version seems hard to find. I would think a slightly longer pulse (120 ns for the 15m version) would just result in a slightly wider pulse on the scope. Looks like you are actually using 110 ns anyway.
@@gkseifert I have the 60Mhz version. Its not clear to me of the lower frequency one would make the pulse less sharp edged. I suspect it would be just as sharp and the different versions are different software/firmware loads - maybe someone reading this knows. But - 120ns pulses would be just fine. Your right - at one point I may have hit the controls and gone from 100ns to 110ns. I just looked on amazon (canada) and there is Koolertron branded 60Mhz one for $152.99 - $10 coupon = $142.99 - $10. The 15Mhz one is $18 less but for that small difference, if you can go for the more 60Mhz.
@@ElectromagneticVideos Thansk again. That Koolertron actually is the same as the JDS6600. The Koolertron site even has the manual for the JDS available for download. In stock and one day shipping too!
Got the 60M Koolertron in one day! It works fine so far, in spite of some of the negative reviews. Perfect for this and anything else I am likely to do. It has an amazingly wide range of capabilities!
Interesting! Just curious, are the scope probes the same length and type on both scope channels? There would be small a propagation delay there, wouldn't there?
Your are right - the probes are different and the cables lengths are similar but not identical. If I was using the difference in time between the signals on the two different probes those differences would have to be measured and accounted for. Since the round trip time is measured by the time difference of pulses shown on the same probe, the difference between the probes is not an issue in the this case. Some commenters have suggested variants of the setup with two photo diodes, one where the light leaves the laser, one where it returns. In that version, identical probes would be helpful and even then I would measures for any detectable differences.
Maybe you could series a second laser diode in the circuit and use it for the shot path. This would allow you to use a focused laser beam for both paths and therefore a sharper response from the photo diode. The diodes being in series should mean that they emit light at exactly the same time, but I’m not sure if there are any other variables introduced by doing this.
That would certainly be another way of doing it to try an improve accuracy. As you said "I’m not sure if there are any other variables introduced by doing this" - like you, I would think the two lasers would emit light at exactly the same time, but maybe because of manufacturing variations, they might not. So one would need to confirm that they are indeed peaking at the same time. Neat idea!
@@lotecque That would be great way to do it. And, whether or not they turn out to fire at the same time, averaging the two results would hopefully produce a more accurate result.
The distance of the long round trip could be multiplied by adding another mirror on the laser / photo diode side of the setup. Instead of hitting the photo diode directly, the light gets reflected back to the distant mirror which then reflects the beam a second time, now hitting the laser diode. Doing this with two mirrors is probably tricky to aim though.
Yes! But your right about aiming. Also, with only a magnifying glass as a collimator, the outgoing beam was quite divergent so a longer (multi mirror or direct) path probbaly would have made the return power too low.
Thanks! The index of refraction of air (excluding humidity) varies almost linearly with pressure. One google results says normal atmospheric pressure is range is from 29.6 to 30.2 inches Hg or about 2%. The index of refraction of air is about 1.0003 meaning in air the speed of light is 0.03% slower than in free space which is well below the accuracy of this experimental setup. Change 1.0003 by 2% and the difference (due to pressure) will be even smaller and so also well below measurement accuracy of the experiment. For humidity - what an interesting question! I did some quick googling and didn't find a good answer. Since the max amount humidity in air (by mass) at reasonable temperatures is around 1 or 2 %, and the index of retraction of most gasses is very close to 1, I would guess the effect of humidity is tiny and also below the accuracy of the experiment. If anyone knows or can point to a humidity vs index of refection graph, please do so!
The important parameter here is the refractive index of the air, which tells you how much the speed of light slows down when going through it. The denser the air is, the slower light goes. The higher the air pressure, and the more water and denser gases like CO2 are in it, the slower light will travel. But the changes are tiny for normal weather variations on Earth. The difference in the speed of light between thin hot, dry, desert air on a mountaintop and thick cold, water saturated air at sea level is only about 0.03%
What megahertz oscilloscope would give a reasonably accurate result. I'd like to try this experiment myself. My scope is a digital 100mhz score with a 1gig sampling rate. What photodiodes would you recommend? (i suppose some respond better/faster than others).
Your scope should be more than enough - the analog scope I did most of the video with was only 50 Mhz. The diode was Advanced Photonix PDB-C156 which I got from Digikey for $2. Its fast and has a large surface area for detection. The hard thing I found about finding photodiodes is many dont have the speed in the specs. I did a DIY video which might be useful to you - lists the parts, goes over some more details: ua-cam.com/video/37Kt1Hs1pOo/v-deo.html
Really simple actually - I did a video about characteristic impedance in transmission lines were the experiment was essentially a TDR : ua-cam.com/video/a-8CaGOmWDU/v-deo.html
This is great, all-round. And Brilliant idea using a bare laser diode and magnifying glasses instead of a laser-pointer, for the sake of beam splitting! I was wondering why you made it so hard on yourself, but now I see the light, twice! And, seriously, 1ft/ns?! How in my 40yrs have I never known this?
Thanks! 1ft/ns is one of the few cases where non-metric just fits perfectly. I find it gives me a better sense of the speed, time and distance than 0.33 m/ns. The other problem with laser pointers is trying to modulate the laser fast enough. I'm sure some pointers are easy to pull apart and get to the laser, but even then I wasn't sure if those cheap lasers can turn on and off quickly enough. Maybe someone who has tried it will tell us. The laser diode I used (Osram PLT5 522EA_Q) is rated at 100Mhz so I was pretty sure it would work.
@@ElectromagneticVideos interesting, I hadn't thought of slow laser diodes, but yeah that does seem plausible to say the least (especially some which might use phosphor?). Slow photodiodes is another reality I've been unexpectedly running into lately. These things we know to be exceptionally fast from common knowledge (fiber optics) aren't necessarily inherently so in all cases!
@@ElectromagneticVideos perhaps the response of the diode is not critical as if there is a delay, both peak will be delayed. This experiment only interested in measuring the difference between two peaks.
@@ericwazhung The problem with forward biased diodes (including lasers and LEDs) is the depletion region is narrow and flooded with charged carriers so to turn it off takes time or those charged carriers to dissipate. I hadn't thought of phosphors in LEDs which also would have a slow decay time. "These things we know to be exceptionally fast from common knowledge (fiber optics) aren't necessarily inherently so in all cases!" So true! I dont know if they directly modulate lasers in the GHz range these days but in the old days one typically used some sort of solid-state light valve for high bandwidth applications. How high speed do you need for your photodiode application? The photodiode I used here was the $2 PDB-C156 - it is rated at having a 15ns response - got it from Digikey. If you needed faster, I also found a PIN photodiode diode from Newark $20 S5973 - rated at 1GHz.
@@manla8397 Yes - as long as one can make out a well enough defined peak or edge on each pulse, as you said its the difference. But when the response time constant gets significantly below the length of the pulse we are using, the max amplitude goes down (not enough time to rise to full value). So there is a limit as to how slow the response can be....
Great video and experiment. I've been wanting to use my oscilloscooes and a laser to measure speed of light for many years. Just haven't got to it yet. I appreciate the insight. Thanks. And you got another subscriber.
Thanks! Great to have you as a subscriber! I will be posting a parts list with the DIY video coming soon. If you want to try it sooner, the laser is PLT5 522EA_Q and the Photo diode is PDB-C156 although I'm sure other will work.
i am assuming you disregarded the fact that light slows down in glass (magnifier) , by a third a think, because the glass is much thinner than the total distance of 114', but in 3' path not so much, could that be the reason for the deviation ?
You right about the speed in glass and disregarding it - it is actually insignificant - with the light delay being 1ns per foot in air, the thickness of all the glass (magnifiers, mirror) combined is less than an inch. So total extra delay from the glass is less than 1/12th of a ns - and even less for the one magnifier we are leaving out when doing the 3 foot round trip. The main reason for the measurement error is really the difficulty in measure the time difference between the 114ft and 3ft round trip peaks on the scope. Noise and the rounding of the peaks being main contributors, as well as me being a bit rushed doing it. It wouldn't take much to improve the accuracy quite bit by taking a bit more care. Th real purpose of these things I do online is to show how simple it can be to do some of these fundamental experiments. A follow up to this one was measuring the wavelength of light a couple of months ago use a laser printed diffraction grating.
If you mean as a detector, I think its positioned to get light directly from the laser and incoming light would be miniscule compared to that. But I was think it could be part of a feedback loop to perhaps (?) increase the modulation bandwidth and make a narrower pulse.
I am so thankful that you made this video since it clears up a nagging knowledge gap I have had since I was a child, plus it is so dang cool. I am planning to reproduce this as I already have all the parts you used. I love how the setup removes the issue of lag in all the components. Just curios what equipment labs use to get down to near zero percent error. Could you please consider making a video on the double slit experiment? I have no idea where to get an affordable device that shoots off one photon (which had to exist 100 years ago).
I so pleased you found it useful - and it is "so dang cool"! I haven't looked into how its done at the level of accuracy but my guess is it is based on Interferometry or interference patterns in some way as part of the measurement. The one photon double slit experiment is something I have been thinking about - don't know how to do it easily yet but it is something I do hope to video sometime! I have been working on part 2 - the DIY version of this video all day today - hopefully will upload tomorrow - take a look - I cover a number of considerations that I skipped over in this video that you may find useful. And please let me know how your version of this experiment went!
Cool setup and explanation, thanks for sharing. Couple of notes / questions: 1. Shouldn't your calculation for dX (or dPath) be 114' - 6' rather than 114' - 3'? Not clear why you take the overall path length for the longer path and just the half path length for the shorter one. Or was the closer mirror actually only 1.5' away from the laser? 2. You were presumably using the speed of light in a vacuum as your baseline, but obviously not testing in a vacuum. The air in the room, the two magnifying glasses, and possibly the glass on the mirror(s) could all be potential sources of error, it might be interesting to take those into consideration. 3. I'm curious why/whether the second mirror is actually necessary. Did you attempt to calculate from the leading edge of the square wave to the leading edge of the "sawtooth" wave? I guess the problem with this is that just because current has started flowing in the laser emitter that doesn't necessarily mean photons are being emitted yet?
Thanks! 1. Poorly explained on my part: 3' was the short round trip path. Mirror positioning was more line 2' from the laser and 1' from the detector. 2. I didn't mention those absolutely valid considerations because as I have been doing these videos I have been learning its best to keep them on the core concept. But since you brought it up, you have just given me great excuse to elaborate: Speed of light c in a vacuum is 299,792,458 m/s actually by definition since m defined based on 1 second and c. The speed of light in air 299,702,547 which is 0.03% slower, so much less than the accuracy of this experiment (since the round trip time is about 100ns, 0.03% of that is 0.03ns ). The glass in the mirrors and magnifying glasses: If the mirrors for the short and long paths were identical, the effects of the mirror glass would cancel when doing the time difference. Similarly if the short return path measurement went though the both magnifying glasses the effects would cancel. However, I used different mirrors and the short return path only went though one magnifying glass. Lets get a worst case sense of glass issue: without measuring the glass thickness, lets assume the total thickness of glass traveled though on the long path (lenses + mirror) is 1 inch. The index of refraction of common glass is usually around 1.5, with some specialty glasses as high as almost 2. So lets use 2 as a worst case. That means the speed of light in the worst case glass is half that of air. Using the 1 foot per ns speed of light approximation, 1 inch though air is about 0.083ns. Through glass, double that: 0.167ns, so the worst case extra delay due to the glass is 0.083ns, well below our ability to measure with this setup. 3. Your right on all accounts: There is a slight lag between laser voltage and photons. We could measure the position of the light pulse relative to the drive voltage pulse with the photo diode right next to the laser, and then subtract that position from the long round trip pulse position. The reason I did the mirror thing was to make it a bit more visually obvious by seeing both pulses at the the same time, the long round trip pulse being clearly delayed. Hope that helps!
How do you know that the speed of light is the same in each direction? You are ALWAYS measuring forward and reflection speeds. The speed 'may' be different in each direction due to other influences in each direction.
Any difference is probably well beyond the accuracy of this experiment. As I'm sure you know, so far (as far as I know) no one has devised a way to measure or prove its the same in both directions. Would be strange and one of those great physics discoveries if it wasn't.
Michelson and Morley pretty much put that to bed in 1887. The speed of light is exactly the same in all directions. en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment
Nice! Did you also measure with the second mirror somewhere halfway to see that the distance between the 2 peaks is about 60 nanoseconds? Just as an extra check that what you see is really what you believe to see instead of working towards a certain result.
I didn't - fair point! Your comment is a great lesson on good scientific experimental setup, verification and procedure for anyone reading this. Would be a good result confirmation particularly if I didnt know the speed of light ahead of time.
Great video. I liked your experiment a lot. I have a thought. If you bounced the laser back and forth between a number of angled mirrors (zig-zag fashion) then that would increase the light path length. In this way, the precise position of the reflected signals peak becomes less significant, so should get you a more accurate result.
Thanks! Yes!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! There should be a spoiler alert in your comment because I was going to suggest that in the upcoming DIY video :) What your suggesting would work really well with a narrow laser beam - this setup spreads the beam quit a bit due to the dollar store optics. But also in the DIY video - some Laser Measurers seem to pulse the narrow beam laser light in way that could be used for this sort of experiment and your suggestion would be perfect for that!
Thanks!!! You know, I never got around to calculating the average : (105ns + 120 ns) = 112.5ns 33.8m/112.5ns = 300,400km/s. Not bad! I probably don't deserve that good a result cause I was a bit sloppy in reading the time delays - too preoccupied with making the video!
Fantastic experiment! Love it when things that we so often take for granted get actually proven/confimed in our basemets. Can't imagine anything more satisfying. Well done, now off to see the part 2. Oh, almost forgot, have you tried a lower resistor valuethan 1k, maybe a 100 ohms? I suspect the rise and fall of the detected pulses will improve. Cheers
Thanks! Yes it was satisfying, particularity due to the challenge I set for myself which was to do it with minimally expensive equipment that anyone could get fairly easily, and also do it an an obvious way. Lower resistor value - your absolutely correct and I considered it. But - the signal out of the PIN diode was so low with the 1K resistor that dropping it to 100 Ohms would have made it unusable. So 100 Ohms would have needed an amp or better optics to provide the diode with more light and in the end I decided it would be better to keep it as simple as possible in it was "just good enough" to do the experiment. Part 2 - really a bunch of ways to consider as alternate ways. The most intriguing to me are using a laser measuring device, some of which do sent out what look like usable pulses, or the mirrored scanner from an old laser printer. I have 3 laser printers hoarded to try that sometime!
@@ElectromagneticVideos Thanks for the response, saw the second part also, very good. You've actually covered some of my questions in it. Re the noise pickup, there are some things you could try: 1. Reduce the battery impedance by bypassing the supply lines right at the cathode of the photodiode and ground end of your 1k sense resistor with some small disc ceramic caps, maybe in parallel with a small 10uF electrolytic capacitor. The ceramic cap can be something like 47 or 100nF. These take care of real high frequencies while the electro the lower end. 2. You may be able to install the complete detector setup (including the battery) inside a grounded metal biscuit tin can, leaving a small window opening for the light to come in. 3. You mentioned the need for an amplifier if the sense resistor was to be dropped from 1k to lower value such as 100 ohm that's only a 10x drop. I've played around with some single transistor JFET amplifiers in a Common Gate configuration (same can be done with normal bipolar junction - BJT - transistors but the biasing of them is more complex than for JFETs) these configurations can achieve frequency responses up in100's of MHz, you might be interested in looking into these. I am aware that the 3rd item above steps into the more complicated category than what your goal to keep things simple is, just thought I would mention it as you never know what ideas can be triggered by doing so. Thanks again and keep up the great work.
@@markg1051 Your welcome! I always appreciate it when someone takes the time to offer some thoughts and ideas! I think the smaller ceramic capacitor would be the way to go - the noise was mostly higher frequencies and putting it in a can would also probably help. JFET for the amp - like the suggestion - nice and simple. If I revisit it sometime I might make a small PCB with the detector and amp all in one, maybe with a metal can for shielding and BNC connector. I have more videos on the way - expect some more soon!
A 60 Mhz scope would be fine. You really shouldn't need more mirrors unless you dont have as long a path for the light to travel as I did. I used 1us sweep rate - so each division on the scope was 100ns. Good luck!
Does the variables of frequency and amplitude come into play? Do we know that light at differnet points on the spectrum all travel at the same speed? This is not to refute...just a question
A very good question. In free space its the same for all frequencies and amplitudes. If you crank though the math the from Maxwell's equations, the speed of light only depends on c = 1/√(με) where μ is the permeability (magnetic constant) and permittivity (electric constant) of free space. Amazingly these can both be determined for static, non-moving and non-time-varying electric and magnetic fields. You will note that amplitude and frequency does not show up in the equations so it does not depend on them, and countless experiments have confirmed this. In matter, its the interactions between the EM fields and the charges in the atoms which alter μ and/or ε. This generally results in the speed slowing down in air, water, glass etc, which why lights bends when it goes through glass etc. Less apparent is change in speed due to frequency and it does vary that way in matter - thats why we see rainbows or a prism splitting light into the colors of the rainbow. Less commonly known is the amplitude effects where which can occur if μ or ε varies with amplitude and all sorts of weird non-linear things happen. More on all of this in future videos! A lot of stuff to squeeze in a short answer!
greate experiment.i think u had to triger the scope with input from the lazer that make it more accturate.and also buildup to cursure to misure the time distance between the two input. just sugesting anyway u made greate job
Thanks! The scope was actually triggered by the signal used to drive the laser. We cant use the trigger as time 0 to measure the round trip delay because of the additional time it takes for the laser to start emitting light, and the measurement delays from the photodiode. The difference between the two peaks is used to account for all those time delays. A cursor as you suggest could be used to measure the time difference between the peaks. However, the real issue in trying to get an accurate measurement is the low signal level from the diode + ambient noise, making it hard to estimate exactly where the peaks were.
Yes! And its easier to do since we have tools like signal generators that can easily generate a pulse or other shaped waveform and we look at it directly on the scope. I'll do that sometime. However, for this video I wanted to actually measure the speed of light with light rather than having to explain that radio waves are just another form of EM radiation and move at the same speed.
@@ElectromagneticVideos does the machine that can generate different wavelengths/frequencies use different energy - volts/amps to generate these different frequencies or is something else happening to generate different frequencies? Thanks for your videos they are so informative and enlightening.
@@ElectromagneticVideos does the machine that can generate different wavelengths/frequencies use different energy - volts/amps to generate these different frequencies or is something else happening to generate different frequencies? Thanks for your videos they are so informative and enlightening.
@@dexter8705 Your most welcome! The most basic signal generator that could be used is an oscillator which can be made with as little as one transistor. The frequency is usually set by a tuned circuit. The tuned circuit can be as simple as a quartz crystal as used in our watches, phones and computers and generate a fixed frequency. If one needs an adjustable frequency, a coil and and adjustable capacitor in parallel can be used (and that's how analog radios tune to different frequencies). Now, if you want computer control and far more versatility in terms of what frequencies - or mix of frequencies - you can generate, a Software Defined Radio can be used such as the Hack RF One greatscottgadgets.com/hackrf/one/ . With that sort of device the possibilities of what you can generate - or receive - is almost limitless. This probably has generated more questions than it answers but hope it helps!
@@ElectromagneticVideos thank you quite informative, I've learnt a little bit how quarts generate a frequency by running certain current through it by learning about it in the system of an atomic clock, but it's great learning about oscillators and interferometers
You could! The speed of light in the cable would be slower than in air bu about 30 or 40%. There are also transmitter and receiver lasers and diodes built specifically for optical fiber so potentially it could be easier to do.
Nice! Why not use a second photo(detector)diode to catch part of the beam just as it leaves the emitter, and use that to create a second trace, then compare the leading edges of the two waveforms? Cheers!
Thanks! You certainly could do exactly as you describe and it would work perfectly, perhaps after verifying that the two detectors are exactly the same speed in case there there are any manufacturing differences - not sure what the variation may be but probably to small to be a problem. The reason I did the 2nd mirror thing was to (try) and make the time delay more obvious by having both peaks on the same scope trace and reducing circuit complexity (not that a 2nd photodetector is complex!). Yes - your right - foot per nanosecond is a bit out - I just like it because it gives an intuitive sense of the speed if on can have an intuitive sense of those speeds! . If it wasn't clear I converted the feet measurement to m before doing the speed calculation.
What a great idea for a video! I will definitely look into into that - what a really would like is tube with a tiny bit of gas in it - just enough to see the electron beam. Not sure if that is easily available ....
@@ElectromagneticVideos It's true. I tried... with a square bottle, I drilled a hole and put an electrode... I did the vacuum with a syringe and it wasn't enough. It has to be a vacuum pump.
@@joelsoncdma Yes - and its a fine line between high enough vacuum for a decent electron beam and and just enough ions for glow and show the beam. I have seen some people online make someone operational CRTs even with low vacuum from a simple pump so its doable. After your comment a few days ago I did some looking. There are acrylic bell jars available for vacuum classroom experiments. Fitted with a thick piece of acrylic at the base it might make a safe way to try making a crt.
I remember doing this way back in high school, 1975, on the football field with a sine modulated Metrologic HeNe laser (goal-post positions) with great results...don't recall the exact results though. THANKS FOR THE GREAT VIDEO... fond memories...
Glad you liked the video and that it brought back some nice memories! You must have had a great since teacher in high school - I was never lucky enough to have done an experiment like that in high school. I was intrigued to hear of a HeNe laser being modulated. Just did some googling and it looks like you can modulated them to about 1MHz. I was surprised you can do it that fast one is dealing with increasing/decreasing ionization in a gas. I'll have to dig out my old HeNe laser sometime ans see if it still works ....
@@ElectromagneticVideos Thanks much for the pleasant reply, The gas laser was actually 10% (IIRC) intensity modulated with an RF amplifier tied into the power supply circuitry (it could even transmit TV pictures as well as audio). Surprisingly, the transmissions were exceptionally clear! I think there are even some of those old METROLOGIC manufactured HeNe's on ebay (long white units). I enjoyed that project, but my real passion was holography. It took some time to get the homemade optical setup done, as well as getting the hang of developing them in my bathroom-converted darkroom. I'll never forget the thrill of being able to make out the reflected glints from my first attempted hologram...from a monopoly hotel roof. Later, the images were so real, that you just had to reach out into thin air attempting to touch the ghost image. Even now the smell of stopbath (acetic acid) brings back those good old memories (silly, I know). BTW I really enjoyed that EM CHAIN! I'll be looking into your other videos, for sure.
Here they are: Laser is PLT5 522EA_Q, Photodetector is PDB-C156. I bought them from Digikey. Mouser seems to have both as well. Careful with variants of the Laser - some are higher power. I would avoid them for the sake of eye safety. Also look into eye safety with lasers if you arnt familiar with the topic - the laser is considerably stronger than a laser pointer and could do damage if used the wrong way. If you do the experiment, please let me know how it went!
@@ElectromagneticVideos Thanks. Why don't you use the rising edge of the laser diode excitation pulse as your time reference and then just use 114' for distance? I mean why do we necessarily need the mirror to create an extra shorter path? Does the laser diode have a significant delay in emitting the light pulse since it receives the electrical pulse from the function generator?
@@amirb715 If you look at the two scope traces in the video, in particular where the the 3' round trip pulse is shown, you can see the 3' round trip pulse starts rising at the beginning of the (upper trace) voltage to the laser, and start dropping when the laser voltage goes to zero. So there is time-constant-like ramp up time for the laser and too (I think) a lesser extent with the photo diode. So the easiest way to take all of this into account is to look at the position of the 3' round trip pulse on the scope, and use that position relative to the 100 foot round trip time to get the 100 foot delay.
well perhaps correcting the calculus with the coefficients of the 2 lenses, you will have better results, since the glass due refraction slows down the light. I was short but you understood what I'm refferring to 😊 what do you think?
To keep the math simple, you assume the thickness of a lens 0.25 inches and absolute worst case index of refraction of 2. Time of flight though air of 0.25 inches = 1 ns/ft x 1/12 ft/inch x 0.25 inch = 0.02ns . With and index of refraction of 2, the time to get through the 0.25 inch is twice that = 0.04 ns. So the extra time to get though the lens is 0.02 ns is so far below the ability to measure time in this setup (+/- 5ns?) it really makes no difference.
@@ElectromagneticVideos yes but at least is aken into consideration and verified its magnitude. However it's great and I can't wati for your next video 🙂
Also keep in mind you did this experiment in Air ( with atmospheric pressure ) and light velocity is slower in air medium so your result might be little closer to accurate.
The index of refraction of air is about 1.0003, so the slowing effect is around 0.03%. For my setup, way smaller effect than the sources of measurement error. But an important point for someone was setting up to try and get sub-percent type accuracy.
Regarding the response time of the photodetector: You are on the right trail asking if it matters. First notice that the top pulse has a more square-like shape, while the bottom trace has a rounded rise and then fall. Technically, this rounding is an exponential shape, and caused by a "time constant" set by the combination of the "100 ohm series resistor", used with the detector, and the "total capacitance." Here the total capacitance equals the capacitance of the detector, plus, the capacitance of the cabling to the oscilloscope, plus, the input capacitance of the oscilloscope. So using different oscilloscopes would change the rounding some. To get a more accurate measurement of light speed, ideally we want the return pulse to have a very sharp rise and fall, giving us a more exact measurement of the time-shift. How to get a measured pulse with a sharper rise time and more accurate measurements? Option 1) Use a "low capacitance" oscilloscope probe; generally these are "divide by 10" meaning they also reduce signal into the oscilloscope by factor of 10., Option 2) Buy a more expensive photodetector rated for lower capacitance and higher speed, 3) Lower the "AC impedance" of the existing resistor by putting an additional capacitor (that has its own resistor in series) in parallel with the existing resistor. From the video, the existing resistor at the detector is 100 ohms. For option 3, putting an additional capacitor value about 10 nanofarads with its own series resistor in its leg of 100 ohms should sharpen the rise time by a factor of 2. Important that the newly added capacitor and resistor are in series with each other, and this pair is across the existing resistor at the detector. A bit technical, but measuring analog signals with an oscilloscope is a window into a level of reality that most never see. Takes time to understand, but pleasing to one with the patience.
@@thomaskoscica7266 I don't think it actually matters in this experimental design. The diode's time constant is, well, constant, due to the constant resistance and capacitance. It will take longer to charge based on the intensity of the incident light, but note that he's actually measuring the difference between the peaks. The peaks are formed when the diode switches from charging (i.e. getting struck by the laser) to discharging (i.e. between laser pulses). It doesn't actually matter how long the diode takes to charge or discharge, the peak will occur at the same offset relative to the laser pulse's removal, and undergo the same "rounding" process, for both the short and the long trip. All that really matters for this experiment is that the two peaks are far enough away from each other (temporally) that it's possible to take a measurement.
@mmaranta785 Two kind viewers already responded to your very valid question, so many thanks to them. The quick answer is, no, because both pulses (from the short and long round trips) are detected by the same diode, so any response delay or lag, or shaping or smearing of the pulses will be the same for both. So the peaks of the pulses may be shifted or smeared, but the relative peak to peak time should still represent the correct difference in round trip times. Hope that helps!
@michaelcalvin42 Thanks so much for responding to @michaelpupeza6692. Your answer really nails it. "I don't think it actually matters in this experimental design" is exactly what I was going for!
Yes! That would give the speed of light in a fiber which would be a bit less than air, bit would be very relevant these days with so much of our data going though so fiber!
It never ceases to boggle my mind how that huge number is actually very slow when you look at the galaxy scale, but also at the computer electronics scale: a 1ns clock pulse is about 30 cm or 1 foot. At those scales you have to literally wait for the light to arrive
Yeah - its so out of our normal life experience of the scale of things. Funny though, as you point out, a 1ns clock pulse is not untypical of computer electronics or comms today and when you work in those areas ns is just a common everyday time scale.
I wonder if a cheap $2 shop semiconductor laser will do the same job. This will be particularly useful to reproduce it in secondary school environments.
I think the main issue is that usually nicely packaged cheap lasers like that include some sort of driver circuitry which probably slows down the on/off response. If one could find one that could easily be hacked, it would be great! When I do the follow up DIY video I'll take your suggestion and ask if anyone can recommend a suitable device like that.
Just a note that the speed of light in air is slightly lower (299,705 km/sec). Not really a factor here. I like the differential delay approach since it cancels out the delays in the laser and opto-detector. Nice job.
We used the Michelson interferometer to measure the speed of light when I was in physics. It is fairly accurate. Seems like we measured it in the lab at 2.99x10^8 m/s.
I'm intrigued! I thought the Michelson interferometer was to measure the minuscule difference in length (or speed of light) between the two legs. Is there some clever trick to using it to measure the speed of of light? You certainly got a great result!
@@ElectromagneticVideos In laser interferometry, a laser source with known frequency is used, and the wavelength of the laser is measured by determining the distance by which the stationary mirror needs to be moved to cause destructive interference. Light's speed is then calculated by multiplying the frequency and wavelength.
I posted in the comments above another way to measure the speed of light using radio waves. It is a very simple experiment involving 2 dipole antennas and transmitting a carrier between then and measuring the phase shift on an oscilloscope as a function of antenna separation. The results reveal the instantaneous nearfield and the light speed fairfield you guys are measuring. My results show that the speed of light is not constant in the nearfield and this has serious repercussion for Relativity theory and science in general, because so much of it today is based on it. The experiment also shows that information can be transmitted faster than light using nearfield electromagnetic fields (light).
This would be a great science demo for junior or high school class.... isn't the speed of light measured in a vacuum? How much would the atmosphere slow it down? Interesting video...
Your right it slows down in air - but only about 0.03% so the difference is way less than the accuracy one would hope to achieve. I do think it would be a very neat experiment for a high school science class. Hopefully some teachers who are good at inspiring their class will see this video!
The higher bandwidth of your scope and probes will increase accuracy. Also consider using a tip ground to shorten your ground path. The coiled wire thing that is about a cm long and slips onto the metal ring at the end of the probe.
By tip ground you are referring to what I would call a ground spring? That certainly might have helped. Biggest issue was noise and that might have reduce it a bit more.
Very nice experiment! Using a delta in distance is a great idea. Oscilloscopes is a fantastic "cheap" tool for realizing such experiments. The only thing I don't understand is the "utility" to use imperial measurements. Fist time I see that in scientific work. 99% of the planet use metric system ;)
Thanks! Its funny how scope are now "cheap" - in the old days they se so expensive they were almost out of reach for many. The reason I use imperial here is I find the "1 foot per nanosecond" makes it really easy to intuitively associate distance with time when one is getting sense of what is going on. And as a result of 90% of the video audience being south of me (ie in the USA ) I also test to often mention imperial measurements in my videos since that's what they are most familiar with.
You could of made it a lot clearer so that uneducated people like me can see exactly what is going on, a schematic would be helpful too, I really appreciate that you took the time and effort to share your knowledge about light speed, please make another video, I would like to know about the speed of light, another point I would like to make is there a difference between sun light and laser light. Thanks for sharing what you know. Cheers from me 😷👍👍👍
I'm working on the DIY video, which includes a schematic. Its always a struggle as to how much to put in the video - too much and any viewers get bored. So I decided try a 2 part video this time so people like yourself who are interested enough to want the details can look at, and other can skip. Will be interesting to see how many look at part 2! And I do have plans sometime for a video about sun (white) light and laser light . Cheers to you too!
@@ElectromagneticVideos Thanks for your reply, me and many others really appreciate it, we will look forward to the next video's on this topic. Cheers from me 😷👍👍👍
Was there really a need to do this by considering the two different paths (3' and 114')? Just the difference between the drive pulse and the arriving pulse (after traversing 114') would have given you the answer. Notably, the 3' signal is indistinguishable in time from the the driving pulse on the scope (within the overall time resolution of the PD, scope, and all electronics between) because 3' is 3ns in time (on scope). By doing the two-paths method, I would say more uncertainty has been introduced into the experimental result. To get better resolution, need to reduce drive pulse width, improve PD speed, and substantially increase path length.
I felt the two path on one trace method makes the "the light really is delayed" a bit more obvious. And yeah, the 3' really makes no difference given the measurement accuracy.
Hi, interesting comment, I just want to check my understanding. Why would decreasing the drive pulse width improve things? I thought more important is how fast the laser diode switches on or off. If there is a wide drive pulse, as long as the switch off time is only 1 or 2 ns, then you can compare the tail of the drive pulse with the tail of the received pulse and the width of the pulses is not important.
@@declankruppa8300 Your absolutely right - if you had a laser (or any slight source) that could be switched incredibly fast and a suitably fast receiver in many ways it would be a much nicer experiment. But those speeds are not easy to do cheaply - I did find a $20 1.5Ghz photodiode that should have ns type resolution, but one would need a high speed amp near the diode and an expensive higher bandwidth scope (my 500 MHz digital scope was $400 used a few years ago), So the by adjusting the drive pulse width to what turns out to be about 100ns in my setup, the combination of the rising and falling time constants of everything combined produces a somewhat triangle like pulse. While the distances used are such that pulses overlap, the peaks are separate and that makes the short and long round trip pulses evident. So shorter, nicely formed pulses are partly useful to make the experiment a bit more obvious to the viewer and the purpose here was so nicely show the speed of light rather than measure it exactly. Hope tat helps!
@@declankruppa8300 Thanks for the comment. You are right in saying how fast the laser turns off is important. If you look at the scope traces shown in the video, the cusp of the PD signal corresponds to the beginning of the fall-off of the laser output. Likewise, where the PD signal begins to rise is where the laser begins to rise. So, there are two places to do the timing. The point about pulse width is that if you keep reducing the laser pulse width (but keep fluence), you would ultimately begin to ALSO improve the rise & fall times of the laser output, and if the PD can keep up with the improved time resolution of the laser, the overall time resolution can be improved. Think of a delta-function laser pulse and the response of the PD to the delta-function laser pulse as yielding the ultimate time resoltuion. As shown in the video, the PD reponse has a significant parasitic capicitance, and a delta-function-like laser pulse would result in an instant rise of the PD signal followed by a decay profile characteristic of the PD. Notably, time-resolved spectroscopy nearly always strives to have short, delta-function-like laser pulses to improve time resolution. Hope this explains better.
Huge fan of your channel. Want to thank you for your exceptional content and answering our questions. I'm curious why you based the measurements on the time between the short and long distances, instead of the time between the source and destination current peaks. Intuition tells me because reception response is the same. So why bother with source current?
Thank you so much! Why use the difference between the two received peaks rather than the difference between the current pulse powering the laser and the received peak from the photodiode? Because I was unsure how much delay there was between the start of the current pulse sent to the laser and when it would start to emit light. And also the response time of the photodiode. Add to that the complication of does the laser response time change as it warms up after a operating for a while? So by comparing the two peaks from short and long light paths, all of those uncertain delays are applied equally to both long and short measurements and shifts their peaks by exactly the same amount. So the only thing we need to know is the difference in round trip paths and difference in peaks times. All the other unknowns cancel themselves out. Hope that makes sense!
@@ElectromagneticVideos I see now. The unknown delays are equal in both measurements, and therefore cancelled when we subtract one from another. Apologies if this was already addressed in part 2, which I plan to watch soon. --Kevin
@@d46512 Exactly! Not sure of I mentioned it in part 2 or not - and I was happy to explain it here. Its actually not uncommon to do things that way - often way easier that trying to characterize some parts of a system with extreme accuracy. Enjoy part 2! If you have any questions or thoughts, don't hesitate t comment and I will try and respond!
You’ve mentioned “round trip” several times. Is it possible outgoing and incoming light travels at different speeds? I don’t know why it would be different but also don’t know why it would be the same. Great video!!!
Veritasium has a video about this. Basically all speed of light measurements are round trip because of the difficulty (impossibility?) of synchronizing clocks without certain assumptions.
Thanks! @chrimony response to your questions is so nicely stated! Your are absolutely right though - while one cant really measure the one way time, it would be very strange of they were different. And contrary to what few commenters have said, technology like GPS works because of our understanding of the speed of light, time and gravity, (Einsteins stuff) and if there were significant weird inconsistencies, well, we would probably have issues with those devices!
Isn't the speed of light ~300,000Km/s in a vacuum? Wouldn't that mean the air in the room slows the light down and thus the analog scope may be closer?
Excellent setup 👍🏼. I believe there is a need for a very large room to remake this experiment. I wonder if the mirror could also be placed outside the house or would there be too much disturbance (natural light etc)? Thanks for sharing your knowledge!
Thanks so much! In terms of room size, it really depends on how accurately you can determine relative positions of the returning light pulses. In my setup, the position of each pulse is probably +/- 5ns, so the difference between the two pulses is probably about +/- 10ns. If we want a +/- 10% accuracy, we need 100ns round trip time = 100 feet of round trip or a 50 foot long room. Even a 25 foot room would work with a reduce accuracy to about +/- 20%. I was able to reflect the laser beam from one end of my basement to the other though a hallway to get the 50 feet. With a cardboard box shielding the receiver photodiode, it would probably work fine outside even in daylight - focusing the laser beam with the lens onto the photodiode really makes it work well. Even with all the lights on in the basement to the point where I could hardly seen the green from the laser. there was no visible degradation to the received signal. However, the big problem doing it outside would be trying to line up the laser, mirror and receiver. You would would probably have to do that at dawn or dusk or even better, at night to make the laser light more visible. One thing I had though of for a shorter room would be to have a mirror at both ends and bounce the light back and forth a few times to increase the distance traveled. Of course its a more complex setup and more alignment issues ....
@@ElectromagneticVideos The idea, having more than one mirror to reduce the room size is very helpful 👍🏼. Thank you very much, I really appreciate your work!
@@ElectromagneticVideos Yes, I will try it in the near future. I already measured the speed of sound, which was much more easier to do. Looking forward to your next videos 😊.
@@gerd3136 Fabulous! When you do please let me know what your result was - hopefully with a bit more care a bit more accurate than mine. How did you do the speed of sound? Do you have it posted anywhere?
Great video! Thank You for sharing this. I think you actually underestimated the difference of time between peaks in the case of digital scope. I understand that this is very hard to do. But I think if you compare "tail" of leading peak, then you should compare "tail" of the following peak. Sorry for this, but I think this is more precise than it seems. If you want I could try to fit your data to some "model" function to make it "more scientific". I want to underline that you put Your results without "messaging the numbers" this is quite a bit of a problem in scientific community to "filter" or "denoise" the data. Thank You.
Thanks! It was hard to get the exact peak position, not only because of noise but also because of the wobbling second mirror I has holding in my hand because I ran out of tripods. "try to fit your data to some "model" function" - yes! Since we know the shape of the pulse, doing least mean square fit with a model pulse to find its best estimated position can not only average out the noise but also produce sub pixel (or time sample) positional accuracy. Even simpler, a "center of mass" type calculation for pulse would improve things as well. As far as presenting the data as is, I think I also mentioned how Galileo declared his results as inconclusive - a great example of reporting a result correctly. I'm a bit more optimistic than you about data being fudged in the scientific community - there have been a few notorious cases in the last few years but it generally gets caught when others get different results.
We can only ever measure the speed of reflected light. What is the speed of reflection? And what is reflection anyways? And is the speed of light independent of the direction of travel?
@@ElectromagneticVideos We could avoid the "reflection issue". Instead of using a mirror we could have similar laser generator sending a separate beam back, that is triggered by the incoming beam. Or a beam that is sine wave modulated and we measure the phase difference. It would be interesting to see if we get the same result as a mirror - but of course the accuracy is nowhere near enough for that.
Another idea: Have two sine wave modulated light sources at each end, same frequency, and then move the detector along the path and measure the distance between phase cancellation.
@@georgedereck6525 Remember in this experiment as done in the video, any possible delays from the mirror are taken in to account by having both short and long light paths include a mirror and taking the time difference between them.
This seems like a good way to calibrate the time base. C is by definition set to 300000km/s exactly. I think they changed the definition of the meter to match the cessium reference
Minor correction : c is defined as 299,792,458 m/s exactly (300000km/s is close enough for most things and is a more convenient number). And a second is from the Cesium reference. So you would probably start with time from an atomic clock and then somehow use that determine a meter length (maybe with interference/standing waves?).
Whether more distance would improve your agreement depends upon the primary source of your error. If it is due to your oscilloscope's timing calibration, then more distance would not help at all. If it is due to uncertainty or error in the distance between the wall and the (near) mirror, then it would help a great deal. If it is due to uncertainty in reading the oscilloscope, then more distance would help. In fact, expanding the time scale would improve this measurement from ~10% to ~1%. Finally, the relevant distance is from the near mirror to the wall, to the detector MINUS the distance between the near mirror and the detector... 2x57' doesn't seem to be accurate.
"If it is due to uncertainty in reading the oscilloscope, then more distance would help." That's exactly it in this case. And mostly as a result of the noise making it a bit hard to see the exact peak position. Looking at the Digital scope I think I misjudged one of the peaks a bit - hindsight is 20/20! Distance - maybe I wasn't too clear - I did subtract 3' from the 2x57' to account for the two different round trip times. The measurements are accurate to a bout a a foot (=1ns) . I didnt worry about any more accuracy since I wasnt expecting to be able to read time to anywhere close to a ns. All great points you make - well worth looking at for anyone trying a version of this measurement!
I would have thought you would have been able to achieve better accuracy. Trace 1 is irrelevant [upper] because it is measuring the pulse to the laser and wasn't used anyway. The discrepancy is in the measured distances and possibly the response time of the photo diode. An alteration of the experiment that might help is to split the bean into two using matched response time photo diodes that have precision measured distances each one catching their respective beams of the split. Also ensure that the wires from each photo diode to the scope are of the same length but send them to two separate traces. That way you can align the traces to the same level and easily measure the time differences between the peaks.
Although I didn't mention it (I have learned to keep things as short as possible the UA-cam audience) I had trace 1 displayed as an easy way to see which pulse (long or short round trip) I was displaying. The downward edge corresponds to the short round trip pulse peak, at least to within a couple of ns. The multiple photo diode approach would sure work. I will mention that option when I do the follow-up DIY video as another way of doing it. My intent here was to keep things as simple/obvious as possible so anyone could understand it and the two pulses on the same trace seemed like a good way to do that. Your absolutely right about getting better accuracy with a bit of work. Noise was a big issue and trying to read the pulse peaks accurately. Some amplification right next to the photo diode would probably have helped, but I wanted to keep things simple.
Yeah but we are never really measuring the speed of light are we? Not directly. Like for example I would measure a popsicle stick. We have only ever come up with a way to measure the round trip and divide by 2?
I would suggest using EME Earth-moon-Earth (bouncing radio waves off the moon) as a final step to increase the accuracy of speed of light measurement. After that it would be way too expensive for any hobbyist to buy a powerful enough laser to bounce off of the retro-reflector left on the moon by Apollo 15. Because of temporal smearing caused by the radio signal bouncing off the curved surface of a sphere the signal peak will be spread out. But should be accurate to less than 1 percent error from the actual speed of light.
The definition of the meter is the distance light travels in 1/299,792,458 of a second in a vacuum. So an accurate measure of light travel time dictates the distance that light traveled in space, INDEPENDENT of your lab, which we both know is in motion in space. Light travels independently of the lab. The light travel TIME is a measure of DISTANCE that light traveled in space, not the distance that the light traveled in your lab which also traveled a distance in the same period of time. Pause the video to view the pic. Notice that in the box the distance from the center of the box to the Z receiver is .5 light seconds in distance, but it takes the light .65 seconds to travel from the center of the box to the Z receiver. Also notice the distance from the center of the box to the X receiver is also .5 light seconds of distance in the box, but it takes 1.384930 seconds for light to reach the x receiver. The distances are the same in the box, and yet light takes 2 different times to reach the receivers. That means light speed in the box is different when measured in the different directions. That makes Einstein's second postulate of Special Relativity false, which makes his theory Bunk! ua-cam.com/video/udPDkq9tT-s/v-deo.html
As in an additive sense? Yes - in almost all common situations. There are some materials that do exhibit nonlinear behavior that can be used in interesting ways: en.wikipedia.org/wiki/Nonlinear_optics
I must have not been the clearest on that - your not the first person asking - its 3 feet round trip. The individual parts are more line 1 ft + 2 ft = 3 ft.
It will be slower - it slows down in denser things like water which is why light bends when it enters water at an angle (future video). Hippolyte Fizeau who first measured the speed of light by sending a pulse of light did did a lot of follow up work measuring the speed of light in water and other materials back then when it was all unknown. Today the relative speeds is given and the index of refraction which for water is 1.33 (so speed of light in free space is 1.33 times faster than in water). You can do the same experiment to see the speed of light (in that case a RF pulse) in a wire which is another case where it it slower. I did that here: ua-cam.com/video/a-8CaGOmWDU/v-deo.html
@@ElectromagneticVideos I was always skeptical of it being a constant & argued with my old Physics Teach that there's No Such Thing as a Closed System in Nature where a Vacuum is Completely Empty of All Matter & Therefore Using it as an Example isn't Valid.
@@Shivaho It actually get even more interesting than that - there are actually two speeds of light - the phase velocity (the speed the waves move) and the group velocity (the speed they carry information or energy). They are the same in free space, but different in many other situations ... also a future video!
@@ElectromagneticVideos Cool I was a Science Geek in High School but the Arrogance of My Teach turned me off to a career in it. Still I'm Fascinated by Space & Science but also find that the Missing Link to Everything is the Nature of Consciousness & it's Impact on Humans Perception of Reality. It's the One Thing Science Can't Seem to Wrap their Heads Around & is Why They Can't Truly Explain the True Nature of the Universe.
@@Shivaho Thats so too bad! I sometimes cant believe I still liked science after many really boring and uninspiring classes in school. Luckily I went on to do it in university. The great thing is all the wonderful science stuff on youtube. Have you see Periodic Videos (chemisty) and Numberphile (math) - some of my favorites!
1 what about probe wires error? in probe wires electricity doesn't Travel at the speed of light. 2 or the scopes processing data. to process speed of light data it needs to be much faster than the speed of light itself. (magnetic field wich moves election rays on the scope can only move it slower than the speed of light) 3 reactance speed of photo diode and laser. 4 rise and fall speed of the pulse generator (thus also affected by the velocity of the probes) the test is awesome. I re-created it in my shack with same results.
The speed of signal in wires is typically a bit less than the speed of light. In most coax cables, its usually between 60% and 90% the speed of light. Since the probe cables are only 2 or 3 feet long there is only a 2 or 3ns cable delay. And since they are about the same length, and we are looking for the difference, the delay times cancel. Scope - it isn't faster than the speed of light since the distance between the two peaks is perhaps 2 cm, way less than the total distance the light travels in the experiment. And even if it was more - the dot on the scope may appear to move to our eyes, but in reality is just where some electrons land. It could in theory move faster than the speed of light because the electrons dont follow the dot position. The generator rise and fall and detector. All cancel, because we look at the outgoing and return pulses with the same detector and probes. So there are undoubtedly delays from those things, but again since its the difference that matters, they cancel. You re-created it! How fantastic! Did you use the same laser and photodiode? Or did you use difference ones? What value for the speed of light did you get?
super i think we must 1st synchronize the oscilloscope with a 10MHz GPSDO (Global Positioning Satelitte Disciplined Oscillator) then we have a better much exact result. Also the long way can be better in case of a more longer distance. But thank you for this cool Video!
As @stargazer7644 indicated, the scope timing issues were really a less significant problem. The biggest issue I found was noise making it hard to get the precise pulse positions. So glad you liked the video! Thanks!
@@stargazer7644 the response time on the photodetector is from the direct beam and the indirect beam I think all the time the same. Maybe it depend on the falling in angle - that this make sense if you run it by a prism. The Analogue Oscilloscope are for sure 10% wrong and also the Digital Oscilloscope is wrong in timing. If you mentioned noise so this could be phasenoise from the Oscilloscope itself - maybe we can install a amplifier after the photodetector this will increase the rising edge then we reduce the noise to. This Generator I use to but the arbitrary signal what come out is also a inhabitable signal so the start point is not correct. Maybe a blue laser light will also make more energy in case of the higher frequency. I think that we can make it more exact if we change some thing - but this is not to understand as a critic! I like your Video very much it clear tell us what happened with the measurement. Thank you again for it!
@@weakbit633An oscilloscope is not ever likely to be 100,000 ppm (10%) out of spec. They are calibrated instruments. I work with them every day and I've never seen one out that far, even after decades without calibration. They're designed to measure signals and they wouldn't be of much use if they couldn't do that. The response time on the photodetector will depend on its capacitance which will ultimately limit its frequency response. Yes it will apply to both beams and add a fixed offset to the timing.
@@stargazer7644 But this is a Philips Oscilloscope this is minimum 10% out of spec (read the manual) and if it is old like this ~50years (this type is from the 70s) then it is totally out of spec! All Analog Oscilloscopes are have a measurement fail from about >10%. A analogue Oscilloscope is not a precision instrument more a estimate Instrument. This Oscilloscope is Service Oscilloscope and not for precision measurement. Also this Generator for €60,- is only a problem in all - this Generator is also not Synchronized and I've this generator is the cheapest instability arbitrary generator. - But I don't criticism the precision of this measurement equipment. You never can read a exact timing like the 120µs from this blur screen this is impossible. Also he mentioned that the noise is some problem. Hahaha calibrated! I know nobody that make a calibration on a service scope! This scope is near 0years old and nobody will make a calibration!
Thanks! "quarter wave resonant frequency based on the wire length" - what wire and how much % higher frequency? The self resonant frequency of any coil (including ones meant to be used as a tesla coils) is the result of the self capacitance of the windings (and of course the inductance). Those stray capacitance can be highly variable based on things like slight changes on winding spacing, wire insulation, coil form permittivity etc so I would not at all be surprised at significant deviations from the resonant frequencies that one might be expecting...
Hey, I measured the time it took to walk to the refrigerator and back with a beer! It was 37.17452 seconds. (That was the resolution of my counter.) :)
And we all know how critical having those last few digits are :) . Actually you sure point out how with digital meters people get fooled into a false sense of accuracy these days.
Its funny, mine is "JDS" but I have seen them with the Koolertron branding. I wonder if one Chinese company is now copying the others stuff, or just rebranding it? Either way, you cant beat what that signal generator can do for the price! May not be as perfect as an HP, but for a bit over a hundred $, wow.
Kind Sir, my "metric to mile" tool, says it's 322,000 km per second. Is that right? For I was taught many yrs ago, that light is 186,000 miles per second. Where did I go wrong kind Sir? I don't use metric yet! LOL.
The speed of light isn't an "accepted value". It is defined since 1983 to be exactly 299,792,458 meters per second. It is kind of odd that you started out in imperial measurements and switched to metric half way through. An interesting comparison would have been if you did the same thing with 114 feet of coax or twisted pair to show the speed of light in wire is much slower than the speed of light in air. That would have been a good way to mention the refractive index of air and the fact that the speed of light in air is slightly less than in a vacuum.
You said, " It is defined since 1983 to be exactly 299,792,458 meters per second." Yes, it is DEFINED by the definition of the meter. Since 1/299,792,458 of a second of light travel time in vacuum is the definition of 1 meter. 599/299,792,458 of a second of light travel time is 599 meters. 3,876,564/299,792,458 of a second of light travel time is 3,876,564 meters. and what do you know...299,792,458/299,792,458 of a second (1 second) is 299,792,458 meters. Since the distance was 299,792,458 meters and the time was 1 second, it is a DEFINED FACT that the speed of light is 299,792,458 meters per second due to the definition of the meter. There is NO POSSIBILITY of the speed of light being any different than 299,792,458 m/s because it is defined. It can't be slower or faster.
@@rmeyers2021Actually, you have that exactly backwards. The length of the meter is defined by the length of the second and the speed of light. It is the distance that light travels in 1/299792458th of a second. They adjusted the length of the meter to fit the defined speed of light in 1983. And yes, the speed of light is defined, hence my use of the word "exactly" and the entire point of my comment.
@@stargazer7644 There is no "length of the second." A second is a unit of measure of TIME. A second is a specific duration of time. The "speed of light" is a SPEED, which is distance/time. There is no "speed of light" until you measure the time of light travel, which then defines the DISTANCE in the unit of measure of "meter". There is simply distance and time. In order to have a unit of distance and measure distance you have to define the unit of measure, which the unit "meter" is defined by the TIME of light travel. As you will notice in the pic in the linked video, the "speed of light" in the box is different in different directions in the box. In the box the distance is the same from center to each of the receivers, but light takes two different TIMES to reach each of the Z and X receivers emitted from the center of the box. Nice try, though. ;) ua-cam.com/video/udPDkq9tT-s/v-deo.html
@@rmeyers2021 FFS man. The second is defined as the time it takes for 9,192,631,770 transitions of the unperturbed ground-state hyperfine transition of the caesium 133 atom. The length of the meter is now tied to this definition. The speed of light is not a measured quantity, it is a defined quantity and is invariant with direction or motion. The length of the meter is not a measured quantity, it is also defined. This has been true since 1983. Nobody likes an asshat.
@@stargazer7644 So how do you know the distance of a meter if you don't measure the light travel time of 1/299,792,458 of a second? Can you explain why it takes .65 seconds for light to travel from the center of the box to the Z receiver, but it takes 1.384930 seconds for that light to reach the X receiver?? I can't wait!!
Since you brought up pico seconds, was just looking ta the manual for one the laser measurers - seems to mention a 600ps pulse - if true thats quite impressive.
Brilliant. Lots of science teachers around the world will be thanking you for your efforts
Thanks! I would be wonderful if some of them did some version of this with their students and inspired them to go into STEM type careers.
Nice experiment, it is also the basis of which TDR (Time Domain Reflectometry) which is used to detect the distance to a break in optical fiber, or coaxial cable for RF.
Thanks!!! And yes TDR. I did a video essentially on that trying to give viewers some sense of reflections on transmission lines and why termination resistance is so important - if your interested, here it is: ua-cam.com/video/a-8CaGOmWDU/v-deo.html
The +-7% error is that primarily caused by your scopes or just where you choose to measure the peaks or blobby light and handheld mirror?
If you used a focused laser and a mounted beam spliting prism would that give sharper peaks and remove a lot of the error?
I think it was more from where I choose to determine the peaks were. Noise in the measurement made it hard, and for the digital scope, when I re-look at the images, I probably read one of the peak positions a bit wrong.
I think your right, a better focused beam would have provided more optical power to the detector diode and a mounted prism or mirror would have helped - me hand-holding the near mirror did not produce the most steady reflection. In fact, with a better mount, one could then average a number of scope traces to produce an almost noiseless result. With a bit of care I think this setup with the approximately 50' distance, getting a result within 2 or 3% is quite realistic.
Excellent set-up and that averages out impressively well. Thanks for sharing your knowledge with us!
Thanks and your welcome!! Yes - the results were good. I think with a low noise amp right at the photodiode once could reduce the noise levels on the scope and hopefully get significantly more accurate individual readings. But this was good enough!
What a neat set-up, I think all schools should do this in science class. Excellent video!
Thanks so much! Yes - really easy to do in the end - would be a great science experiment.
It was very interesting, but what about the slow down on light velocity produced by the lents glass ?
the short path doesn't pass at least one of the lents.
Your right about the short path missing a lens, so ideally we would put a lens somewhere in the second path so it equals the lenses in the long path. But - the reason I could "cheat" is we "know" that light going though glass slows down to perhaps half is speed in free space. The speed in airor free space is about 1 nano second per foot or 1 nano second for 30 cm. The lens is less that 1 cm thick at the thickest point, so in free air it would take 1/30 ns for do that distance, or in the less glass at half its normal speed, 2/30 ns. So removing the lens removes 1/30 ns from the round trip measurement, which is way less than the round trip time and way smaller than the accuracy we can obtain with this experimental setup. So removing one lens has no effect for a measurement/experiment that is as crude as this one.
Now we really should prove this - that is measure how much light slows down in glass. Thats a future video!
Thanks for the question!
It was interesting to see how easily a measurement can be made, thanks.
It's also nice that you're holding a stable lantern that I also happen to own.
I was quit pleased how simple it was to do in the end after some experimentation. Stable lantern - I didnt know it was a stable lantern. It is actually really old - when I was a kid a we lived in Johannesburg where my dad was posted for a few years. We found that lantern buried in an old compost heap in the garden of the house we were living in. The property had been a farm, so stable lantern makes complete sense. If you look closely you can see the writing on bottom part that says "City of Johannesburg".
@@ElectromagneticVideos Wow, thanks for letting me know!
@@emilalmberg1096 Your welcome Emil!
Thanks a lot for the very thorough explanation of your setup. My son (4th grade) is learning about light in tech class this week, so I will definitely try and replicate your experiment at home!
So glad you liked it - would be fantastic if you can replicate it for your son - maybe even show his class? If you havnt seen it, I did a followup with more details about the parts I used and possible alternatives: ua-cam.com/video/37Kt1Hs1pOo/v-deo.html
If you do it, please post how it worked and any changes you may have made to make it easier to do or better!
Looking foreward to the next video. I'll definitly recreate that setup! Thanks for the insightful explanation!
Thanks! Please do recreate it! It would be wonderful to hear your results when you do it and any improvements or simplifications of the experimental setup you may come up with. I hope the DIY video with be up in a week or so, time permitting....
Very nice - very well done. Good design that rules out most alternative explanations, using easily obtained materials.
Thanks! The fun challenge was actually to try and do it with inexpensive and easy to obtain stuff.
Wonderfully conceived and superbly executed; concise and inspiring. Thanks!
Thank you so much! If you liked that, you might enjoy my wavelength of light video where I laser print a diffraction grating to do it: ua-cam.com/video/WwQXQrm33JM/v-deo.html
Perhaps you could make a calibration on the turn on delay of the laser diode and detector by putting them face to face as there may be a delays in these two parts.
Certainly - do that, note the photo detector delay (lower trace) from the upper trace (voltage to the laser) , and repeat with the return signal relative to the upper trace and take the difference. I'm detecting both outgoing and incoming light pulses on the same trace, so the difference in their peaks eliminates the effects of any laser turn on delays.
That Philips Scope looks exactly like the model I was using back in the day (1983) at GEC Coventry...setting up 11GHz QPSK modulators for the system x telephone systen in the UK
You nailed the time frame of that scope. Its a scope from the undergrad labs at the university I was a student at in the early 80s. Great scopes but the push buttons tended to wear out with daily lab use.
11GHz QPSK back then - wow - that must have been incredible at that time! Microwave links?
@@ElectromagneticVideos
It was a time consuming task...With Microwave substrates and pushing around Gold 1 millimeter blocks to tune the Modulators in 0 - 90 - 180 - 270 degree phases.
The Philips Scope was used to tune the modulators at the frequency range (around 11 GHz +/- 0.5 GHz ish) it traced out a datum straight line and you had a second trace which you had to get to follow the datum line,
The setup must have used an equivelent, maybe DC voltage to indicate the Modulators response across the range and the scope displayed this. The frequency range spanned the width of the scope display.
When you got it right, the two lines would be as close to one another as you could get. You then mixed an apoxy glue to fix the Gold blocks in place. To add to this, there were 5 magnets you could twist to help the tuning and they were glued in place with Silastic (This stank terrible).... When you thought you had it right, the Modulator would go into a small oven to cure the Silastic and epoxy. A couple of hours later you retrieved the Modulator and rechecked it....if the magnets or epoxy had moved and the scope trace was no longer following the datum trace within spec across the phases, then everything would have had to be redone, cleaning things before re-trying.
Two a day was all you could hope for.
I was just about 20 and didn't like it too much but it paid well.... 😁
Memory is a little fuzzy I think I remembered it ok.
When I left GEC years later, 19GHz was coming in. While 2 GHz was old tech and the waveguides were about 2 inches wide internally, 11 GHz was about a third of that and 19 GHz was a lot smaller too.
Excellent demonstration with a very clean setup that eliminates a lot of pesky variables.
Thanks! Nothing worse than pesky variables running around in a lab!
Wow, what an insightful video! Watching it has inspired me to try out this technique myself. I've actually experimented with a similar method before, using ultrasonic transducers linked to a dual beam scope to measure the speed of sound. Aligning the source and received signals in phase on the dual channel scope and marking their positions on a ruler scale was quite an interesting process. Then, by adjusting the receiver transducer and noting the distance when the phase match occurred again, I could calculate the speed of sound. Additionally, I've utilized a comparable setup with a time-domain reflectometer (TDR) for measuring coaxial length or velocity factor using nanosecond pulses.
The idea of applying this method to measure the speed of light never crossed my mind before. The historical context, especially the cog wheel method, struck me as particularly ingenious. Thank you for sharing such an informative and well-presented video!
So glad you liked it! If your going to do it, order one one of the laser diodes sooner rather than later - they seem to be in the process of being discontinued. Finding a diode with suitable specs (acceptable brightness, speed and cost) was not easy although there definitely others in production. Very cool you did the audio version!
TDR - that was my favorite EM lab when I was a student. Actually did a video on it- disguised as "why doesnt a 75 Ohm cable measure 75 ohms".
Alright, I came to this video expecting it would be a waste of time, but you've honestly outdone yourself. This was a very interesting learning experience and although I'm sure we could agree that there may still be some potentially unknown variables in the setup we can definitely somewhat easily and convincingly measure a clear and reasonable result in this way.
Wow - thank you so much! Yes there are things that a purposely did not mention in the video to keep it a reasonable length and not clutter the core concepts. I'm working on a "part 2" with more details how to DIY and things that should be considered.
Well spoken, articulate & well presented.
Thanks! I appreciate that!
01:58 does roating cogwheel bend light or just block (light hits hill) or pass through (light goes between hills) light?
It just blocks it to make a beam that rapidly goes on and off. Same idea as what I did, except I used modern electronics to switch a laser on an off even faster so the experiment would be easy to do over a much shorter distance.
Fantastic! I had always assumed that it would be completely impossible to show a delay without hundreds of thousands of dollars in research grants and a fully equiped lab. Heck, I will be able to do this in my own basement with equipment I already have. Please do post that follow up video. I will be waiting, but I am sure I can work it out myself since you have shown it to be so simple. Thank you!
So glad you liked it! I'm working on the follow up video, and will include suggestions from viewers of this one. I would be thrilled if you do it and let me know the results and any improvements or simplifications to the experimental setup you might do. If you dont already have a photo diode and laser, I used PDB-C156 and PLT5 522EA_Q which I got from Digikey, but I think Mouser, Newark and other also have them. I di want to see if I can find a fast, bright LED to use in place of the laser to make things a bit safer.
@@ElectromagneticVideos Thanks, I do need the laser diode.
One other question. I do need a real signal generator as well (but they are not expensive and I wanted one anyway). The JDS6600 comes in a number of versions up to 60 Mhs. Which model are you using? It looks like the 30M would be needed to get down to a 100ns pulse according to the specs, but that version seems hard to find. I would think a slightly longer pulse (120 ns for the 15m version) would just result in a slightly wider pulse on the scope. Looks like you are actually using 110 ns anyway.
@@gkseifert I have the 60Mhz version. Its not clear to me of the lower frequency one would make the pulse less sharp edged. I suspect it would be just as sharp and the different versions are different software/firmware loads - maybe someone reading this knows. But - 120ns pulses would be just fine. Your right - at one point I may have hit the controls and gone from 100ns to 110ns.
I just looked on amazon (canada) and there is Koolertron branded 60Mhz one for $152.99 - $10 coupon = $142.99 - $10. The 15Mhz one is $18 less but for that small difference, if you can go for the more 60Mhz.
@@ElectromagneticVideos Thansk again. That Koolertron actually is the same as the JDS6600. The Koolertron site even has the manual for the JDS available for download. In stock and one day shipping too!
Got the 60M Koolertron in one day! It works fine so far, in spite of some of the negative reviews. Perfect for this and anything else I am likely to do. It has an amazingly wide range of capabilities!
Interesting! Just curious, are the scope probes the same length and type on both scope channels? There would be small a propagation delay there, wouldn't there?
Your are right - the probes are different and the cables lengths are similar but not identical. If I was using the difference in time between the signals on the two different probes those differences would have to be measured and accounted for. Since the round trip time is measured by the time difference of pulses shown on the same probe, the difference between the probes is not an issue in the this case. Some commenters have suggested variants of the setup with two photo diodes, one where the light leaves the laser, one where it returns. In that version, identical probes would be helpful and even then I would measures for any detectable differences.
Maybe you could series a second laser diode in the circuit and use it for the shot path. This would allow you to use a focused laser beam for both paths and therefore a sharper response from the photo diode. The diodes being in series should mean that they emit light at exactly the same time, but I’m not sure if there are any other variables introduced by doing this.
That would certainly be another way of doing it to try an improve accuracy. As you said "I’m not sure if there are any other variables introduced by doing this" - like you, I would think the two lasers would emit light at exactly the same time, but maybe because of manufacturing variations, they might not. So one would need to confirm that they are indeed peaking at the same time. Neat idea!
@@lotecque That would be great way to do it. And, whether or not they turn out to fire at the same time, averaging the two results would hopefully produce a more accurate result.
The distance of the long round trip could be multiplied by adding another mirror on the laser / photo diode side of the setup.
Instead of hitting the photo diode directly, the light gets reflected back to the distant mirror which then reflects the beam a second time, now hitting the laser diode.
Doing this with two mirrors is probably tricky to aim though.
Yes! But your right about aiming. Also, with only a magnifying glass as a collimator, the outgoing beam was quite divergent so a longer (multi mirror or direct) path probbaly would have made the return power too low.
I like your experiment. Thank you. I wondering if air pressure and humidity contribute to the error. Just my guess.
Thanks!
The index of refraction of air (excluding humidity) varies almost linearly with pressure. One google results says normal atmospheric pressure is range is from 29.6 to 30.2 inches Hg or about 2%. The index of refraction of air is about 1.0003 meaning in air the speed of light is 0.03% slower than in free space which is well below the accuracy of this experimental setup. Change 1.0003 by 2% and the difference (due to pressure) will be even smaller and so also well below measurement accuracy of the experiment.
For humidity - what an interesting question! I did some quick googling and didn't find a good answer. Since the max amount humidity in air (by mass) at reasonable temperatures is around 1 or 2 %, and the index of retraction of most gasses is very close to 1, I would guess the effect of humidity is tiny and also below the accuracy of the experiment.
If anyone knows or can point to a humidity vs index of refection graph, please do so!
The important parameter here is the refractive index of the air, which tells you how much the speed of light slows down when going through it. The denser the air is, the slower light goes. The higher the air pressure, and the more water and denser gases like CO2 are in it, the slower light will travel. But the changes are tiny for normal weather variations on Earth.
The difference in the speed of light between thin hot, dry, desert air on a mountaintop and thick cold, water saturated air at sea level is only about 0.03%
@@ElectromagneticVideos thank you
@@stargazer7644 thank you
@@stargazer7644 So that really puts a some bound on what the effects of humidity might be!
What megahertz oscilloscope would give a reasonably accurate result. I'd like to try this experiment myself. My scope is a digital 100mhz score with a 1gig sampling rate. What photodiodes would you recommend? (i suppose some respond better/faster than others).
Your scope should be more than enough - the analog scope I did most of the video with was only 50 Mhz. The diode was Advanced Photonix PDB-C156 which I got from Digikey for $2. Its fast and has a large surface area for detection. The hard thing I found about finding photodiodes is many dont have the speed in the specs.
I did a DIY video which might be useful to you - lists the parts, goes over some more details: ua-cam.com/video/37Kt1Hs1pOo/v-deo.html
@@ElectromagneticVideos Many thanks!
@@NoosaHeads Your welcome! Let me know how it went!
Really interesting set up and explanation. 😃
Thanks you so much! I appreciate that!
Cool! How hard would it be to jerry-rig a time domain reflectometer?
Really simple actually - I did a video about characteristic impedance in transmission lines were the experiment was essentially a TDR : ua-cam.com/video/a-8CaGOmWDU/v-deo.html
@@ElectromagneticVideos Thanks!
You just watched him do it.
@@buidelrat132 Enjoy!
This is great, all-round. And Brilliant idea using a bare laser diode and magnifying glasses instead of a laser-pointer, for the sake of beam splitting! I was wondering why you made it so hard on yourself, but now I see the light, twice!
And, seriously, 1ft/ns?! How in my 40yrs have I never known this?
Thanks! 1ft/ns is one of the few cases where non-metric just fits perfectly. I find it gives me a better sense of the speed, time and distance than 0.33 m/ns.
The other problem with laser pointers is trying to modulate the laser fast enough. I'm sure some pointers are easy to pull apart and get to the laser, but even then I wasn't sure if those cheap lasers can turn on and off quickly enough. Maybe someone who has tried it will tell us. The laser diode I used (Osram PLT5 522EA_Q) is rated at 100Mhz so I was pretty sure it would work.
@@ElectromagneticVideos interesting, I hadn't thought of slow laser diodes, but yeah that does seem plausible to say the least (especially some which might use phosphor?). Slow photodiodes is another reality I've been unexpectedly running into lately. These things we know to be exceptionally fast from common knowledge (fiber optics) aren't necessarily inherently so in all cases!
@@ElectromagneticVideos perhaps the response of the diode is not critical as if there is a delay, both peak will be delayed. This experiment only interested in measuring the difference between two peaks.
@@ericwazhung The problem with forward biased diodes (including lasers and LEDs) is the depletion region is narrow and flooded with charged carriers so to turn it off takes time or those charged carriers to dissipate. I hadn't thought of phosphors in LEDs which also would have a slow decay time.
"These things we know to be exceptionally fast from common knowledge (fiber optics) aren't necessarily inherently so in all cases!" So true! I dont know if they directly modulate lasers in the GHz range these days but in the old days one typically used some sort of solid-state light valve for high bandwidth applications. How high speed do you need for your photodiode application? The photodiode I used here was the $2 PDB-C156 - it is rated at having a 15ns response - got it from Digikey. If you needed faster, I also found a PIN photodiode diode from Newark $20 S5973 - rated at 1GHz.
@@manla8397 Yes - as long as one can make out a well enough defined peak or edge on each pulse, as you said its the difference. But when the response time constant gets significantly below the length of the pulse we are using, the max amplitude goes down (not enough time to rise to full value). So there is a limit as to how slow the response can be....
Great video and experiment. I've been wanting to use my oscilloscooes and a laser to measure speed of light for many years. Just haven't got to it yet. I appreciate the insight. Thanks. And you got another subscriber.
Thanks! Great to have you as a subscriber! I will be posting a parts list with the DIY video coming soon. If you want to try it sooner, the laser is PLT5 522EA_Q and the Photo diode is PDB-C156 although I'm sure other will work.
i am assuming you disregarded the fact that light slows down in glass (magnifier) , by a third a think, because the glass is much thinner than the total distance of 114', but in 3' path not so much, could that be the reason for the deviation ?
You right about the speed in glass and disregarding it - it is actually insignificant - with the light delay being 1ns per foot in air, the thickness of all the glass (magnifiers, mirror) combined is less than an inch. So total extra delay from the glass is less than 1/12th of a ns - and even less for the one magnifier we are leaving out when doing the 3 foot round trip.
The main reason for the measurement error is really the difficulty in measure the time difference between the 114ft and 3ft round trip peaks on the scope. Noise and the rounding of the peaks being main contributors, as well as me being a bit rushed doing it. It wouldn't take much to improve the accuracy quite bit by taking a bit more care.
Th real purpose of these things I do online is to show how simple it can be to do some of these fundamental experiments. A follow up to this one was measuring the wavelength of light a couple of months ago use a laser printed diffraction grating.
usually the third lead on the laser diode is a built-in photodiode - you can use it
If you mean as a detector, I think its positioned to get light directly from the laser and incoming light would be miniscule compared to that. But I was think it could be part of a feedback loop to perhaps (?) increase the modulation bandwidth and make a narrower pulse.
I am so thankful that you made this video since it clears up a nagging knowledge gap I have had since I was a child, plus it is so dang cool. I am planning to reproduce this as I already have all the parts you used. I love how the setup removes the issue of lag in all the components.
Just curios what equipment labs use to get down to near zero percent error.
Could you please consider making a video on the double slit experiment? I have no idea where to get an affordable device that shoots off one photon (which had to exist 100 years ago).
I so pleased you found it useful - and it is "so dang cool"!
I haven't looked into how its done at the level of accuracy but my guess is it is based on Interferometry or interference patterns in some way as part of the measurement.
The one photon double slit experiment is something I have been thinking about - don't know how to do it easily yet but it is something I do hope to video sometime!
I have been working on part 2 - the DIY version of this video all day today - hopefully will upload tomorrow - take a look - I cover a number of considerations that I skipped over in this video that you may find useful. And please let me know how your version of this experiment went!
Cool setup and explanation, thanks for sharing. Couple of notes / questions:
1. Shouldn't your calculation for dX (or dPath) be 114' - 6' rather than 114' - 3'? Not clear why you take the overall path length for the longer path and just the half path length for the shorter one. Or was the closer mirror actually only 1.5' away from the laser?
2. You were presumably using the speed of light in a vacuum as your baseline, but obviously not testing in a vacuum. The air in the room, the two magnifying glasses, and possibly the glass on the mirror(s) could all be potential sources of error, it might be interesting to take those into consideration.
3. I'm curious why/whether the second mirror is actually necessary. Did you attempt to calculate from the leading edge of the square wave to the leading edge of the "sawtooth" wave? I guess the problem with this is that just because current has started flowing in the laser emitter that doesn't necessarily mean photons are being emitted yet?
Thanks!
1. Poorly explained on my part: 3' was the short round trip path. Mirror positioning was more line 2' from the laser and 1' from the detector.
2. I didn't mention those absolutely valid considerations because as I have been doing these videos I have been learning its best to keep them on the core concept. But since you brought it up, you have just given me great excuse to elaborate:
Speed of light c in a vacuum is 299,792,458 m/s actually by definition since m defined based on 1 second and c. The speed of light in air 299,702,547 which is 0.03% slower, so much less than the accuracy of this experiment (since the round trip time is about 100ns, 0.03% of that is 0.03ns ).
The glass in the mirrors and magnifying glasses: If the mirrors for the short and long paths were identical, the effects of the mirror glass would cancel when doing the time difference. Similarly if the short return path measurement went though the both magnifying glasses the effects would cancel. However, I used different mirrors and the short return path only went though one magnifying glass.
Lets get a worst case sense of glass issue: without measuring the glass thickness, lets assume the total thickness of glass traveled though on the long path (lenses + mirror) is 1 inch. The index of refraction of common glass is usually around 1.5, with some specialty glasses as high as almost 2. So lets use 2 as a worst case. That means the speed of light in the worst case glass is half that of air. Using the 1 foot per ns speed of light approximation, 1 inch though air is about 0.083ns. Through glass, double that: 0.167ns, so the worst case extra delay due to the glass is 0.083ns, well below our ability to measure with this setup.
3. Your right on all accounts: There is a slight lag between laser voltage and photons. We could measure the position of the light pulse relative to the drive voltage pulse with the photo diode right next to the laser, and then subtract that position from the long round trip pulse position.
The reason I did the mirror thing was to make it a bit more visually obvious by seeing both pulses at the the same time, the long round trip pulse being clearly delayed.
Hope that helps!
@@ElectromagneticVideos perfectly clear, thanks!
@@MichaelLeonard Great!
How do you know that the speed of light is the same in each direction? You are ALWAYS measuring forward and reflection speeds. The speed 'may' be different in each direction due to other influences in each direction.
Any difference is probably well beyond the accuracy of this experiment. As I'm sure you know, so far (as far as I know) no one has devised a way to measure or prove its the same in both directions. Would be strange and one of those great physics discoveries if it wasn't.
Michelson and Morley pretty much put that to bed in 1887. The speed of light is exactly the same in all directions. en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment
Your setup smartly cancels out the photo diode response time, I would just rely on the transmitted-received time ... well done!
Thanks!
Nice! Did you also measure with the second mirror somewhere halfway to see that the distance between the 2 peaks is about 60 nanoseconds? Just as an extra check that what you see is really what you believe to see instead of working towards a certain result.
I didn't - fair point! Your comment is a great lesson on good scientific experimental setup, verification and procedure for anyone reading this. Would be a good result confirmation particularly if I didnt know the speed of light ahead of time.
Great video. I liked your experiment a lot. I have a thought. If you bounced the laser back and forth between a number of angled mirrors (zig-zag fashion) then that would increase the light path length. In this way, the precise position of the reflected signals peak becomes less significant, so should get you a more accurate result.
Thanks! Yes!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! There should be a spoiler alert in your comment because I was going to suggest that in the upcoming DIY video :) What your suggesting would work really well with a narrow laser beam - this setup spreads the beam quit a bit due to the dollar store optics. But also in the DIY video - some Laser Measurers seem to pulse the narrow beam laser light in way that could be used for this sort of experiment and your suggestion would be perfect for that!
One other great job well done...if using half variations between 105 and 120 ns has accuracy of light speed. Congratulation! ...Thanks
Thanks!!! You know, I never got around to calculating the average : (105ns + 120 ns) = 112.5ns 33.8m/112.5ns = 300,400km/s. Not bad! I probably don't deserve that good a result cause I was a bit sloppy in reading the time delays - too preoccupied with making the video!
Informative and interesting. Thank you.
Well thank you! Glad you found it interesting!
that is awesome! many thanks for your effort, I really enjoyed watching!
Thanks! It was fun to do. I just working on the followup DIY video .....
Fantastic experiment! Love it when things that we so often take for granted get actually proven/confimed in our basemets.
Can't imagine anything more satisfying.
Well done, now off to see the part 2.
Oh, almost forgot, have you tried a lower resistor valuethan 1k, maybe a 100 ohms?
I suspect the rise and fall of the detected pulses will improve.
Cheers
Thanks! Yes it was satisfying, particularity due to the challenge I set for myself which was to do it with minimally expensive equipment that anyone could get fairly easily, and also do it an an obvious way.
Lower resistor value - your absolutely correct and I considered it. But - the signal out of the PIN diode was so low with the 1K resistor that dropping it to 100 Ohms would have made it unusable. So 100 Ohms would have needed an amp or better optics to provide the diode with more light and in the end I decided it would be better to keep it as simple as possible in it was "just good enough" to do the experiment.
Part 2 - really a bunch of ways to consider as alternate ways. The most intriguing to me are using a laser measuring device, some of which do sent out what look like usable pulses, or the mirrored scanner from an old laser printer. I have 3 laser printers hoarded to try that sometime!
@@ElectromagneticVideos
Thanks for the response, saw the second part also, very good. You've actually covered some of my questions in it.
Re the noise pickup, there are some things you could try:
1. Reduce the battery impedance by bypassing the supply lines right at the cathode of the photodiode and ground end of your 1k sense resistor with some small disc ceramic caps, maybe in parallel with a small 10uF electrolytic capacitor. The ceramic cap can be something like 47 or 100nF. These take care of real high frequencies while the electro the lower end.
2. You may be able to install the complete detector setup (including the battery) inside a grounded metal biscuit tin can, leaving a small window opening for the light to come in.
3. You mentioned the need for an amplifier if the sense resistor was to be dropped from 1k to lower value such as 100 ohm that's only a 10x drop. I've played around with some single transistor JFET amplifiers in a Common Gate configuration (same can be done with normal bipolar junction - BJT - transistors but the biasing of them is more complex than for JFETs) these configurations can achieve frequency responses up in100's of MHz, you might be interested in looking into these.
I am aware that the 3rd item above steps into the more complicated category than what your goal to keep things simple is, just thought I would mention it as you never know what ideas can be triggered by doing so.
Thanks again and keep up the great work.
@@markg1051 Your welcome! I always appreciate it when someone takes the time to offer some thoughts and ideas!
I think the smaller ceramic capacitor would be the way to go - the noise was mostly higher frequencies and putting it in a can would also probably help.
JFET for the amp - like the suggestion - nice and simple. If I revisit it sometime I might make a small PCB with the detector and amp all in one, maybe with a metal can for shielding and BNC connector.
I have more videos on the way - expect some more soon!
Your labeled dimension of the short path was not multiplied by 2 - is 3 feet the total?
It is 3 feet in total - I guess I wasn't too clear about that!
Brilliant. Thank you for such an informative video.
Glad you liked it - thats actually one of my favorite videos!
Can I do the same with a tek 60 mhz ,if I use more mirrors ?what is de time base of your setup?
A 60 Mhz scope would be fine. You really shouldn't need more mirrors unless you dont have as long a path for the light to travel as I did. I used 1us sweep rate - so each division on the scope was 100ns. Good luck!
Wonderful video.
Thank you so much - so glad you liked it!
Does the variables of frequency and amplitude come into play? Do we know that light at differnet points on the spectrum all travel at the same speed? This is not to refute...just a question
A very good question. In free space its the same for all frequencies and amplitudes. If you crank though the math the from Maxwell's equations, the speed of light only depends on c = 1/√(με) where μ is the permeability (magnetic constant) and permittivity (electric constant) of free space. Amazingly these can both be determined for static, non-moving and non-time-varying electric and magnetic fields. You will note that amplitude and frequency does not show up in the equations so it does not depend on them, and countless experiments have confirmed this.
In matter, its the interactions between the EM fields and the charges in the atoms which alter μ and/or ε. This generally results in the speed slowing down in air, water, glass etc, which why lights bends when it goes through glass etc. Less apparent is change in speed due to frequency and it does vary that way in matter - thats why we see rainbows or a prism splitting light into the colors of the rainbow. Less commonly known is the amplitude effects where which can occur if μ or ε varies with amplitude and all sorts of weird non-linear things happen.
More on all of this in future videos! A lot of stuff to squeeze in a short answer!
Great video. I love the Philips analog scope!
Thanks! They were a great series of scopes. Its my go-to analog scope!
greate experiment.i think u had to triger the scope with input from the lazer that make it more accturate.and also buildup to cursure to misure the time distance between the two input. just sugesting anyway u made greate job
Thanks! The scope was actually triggered by the signal used to drive the laser. We cant use the trigger as time 0 to measure the round trip delay because of the additional time it takes for the laser to start emitting light, and the measurement delays from the photodiode. The difference between the two peaks is used to account for all those time delays. A cursor as you suggest could be used to measure the time difference between the peaks. However, the real issue in trying to get an accurate measurement is the low signal level from the diode + ambient noise, making it hard to estimate exactly where the peaks were.
Can this test be done with Radio waves?
Yes! And its easier to do since we have tools like signal generators that can easily generate a pulse or other shaped waveform and we look at it directly on the scope. I'll do that sometime. However, for this video I wanted to actually measure the speed of light with light rather than having to explain that radio waves are just another form of EM radiation and move at the same speed.
@@ElectromagneticVideos does the machine that can generate different wavelengths/frequencies use different energy - volts/amps to generate these different frequencies or is something else happening to generate different frequencies? Thanks for your videos they are so informative and enlightening.
@@ElectromagneticVideos does the machine that can generate different wavelengths/frequencies use different energy - volts/amps to generate these different frequencies or is something else happening to generate different frequencies? Thanks for your videos they are so informative and enlightening.
@@dexter8705 Your most welcome! The most basic signal generator that could be used is an oscillator which can be made with as little as one transistor. The frequency is usually set by a tuned circuit. The tuned circuit can be as simple as a quartz crystal as used in our watches, phones and computers and generate a fixed frequency. If one needs an adjustable frequency, a coil and and adjustable capacitor in parallel can be used (and that's how analog radios tune to different frequencies).
Now, if you want computer control and far more versatility in terms of what frequencies - or mix of frequencies - you can generate, a Software Defined Radio can be used such as the Hack RF One greatscottgadgets.com/hackrf/one/ . With that sort of device the possibilities of what you can generate - or receive - is almost limitless.
This probably has generated more questions than it answers but hope it helps!
@@ElectromagneticVideos thank you quite informative, I've learnt a little bit how quarts generate a frequency by running certain current through it by learning about it in the system of an atomic clock, but it's great learning about oscillators and interferometers
Could you use fiber optic cable?
You could! The speed of light in the cable would be slower than in air bu about 30 or 40%. There are also transmitter and receiver lasers and diodes built specifically for optical fiber so potentially it could be easier to do.
Nice! Why not use a second photo(detector)diode to catch part of the beam just as it leaves the emitter, and use that to create a second trace, then compare the leading edges of the two waveforms? Cheers!
BTW, your figure for the speed of light--1 foot per nanosecond--is 1.7% out. That would make one or the other of your figures closer!
Thanks! You certainly could do exactly as you describe and it would work perfectly, perhaps after verifying that the two detectors are exactly the same speed in case there there are any manufacturing differences - not sure what the variation may be but probably to small to be a problem. The reason I did the 2nd mirror thing was to (try) and make the time delay more obvious by having both peaks on the same scope trace and reducing circuit complexity (not that a 2nd photodetector is complex!).
Yes - your right - foot per nanosecond is a bit out - I just like it because it gives an intuitive sense of the speed if on can have an intuitive sense of those speeds! . If it wasn't clear I converted the feet measurement to m before doing the speed calculation.
@@ElectromagneticVideos 👍👍
@@sciencegeekgrandpa8 👍
If you can, please make video of one tranparente cathode ray tube Crt to show how ociloscope woks display waves in realtime.
What a great idea for a video! I will definitely look into into that - what a really would like is tube with a tiny bit of gas in it - just enough to see the electron beam. Not sure if that is easily available ....
@@ElectromagneticVideos It's true. I tried... with a square bottle, I drilled a hole and put an electrode... I did the vacuum with a syringe and it wasn't enough. It has to be a vacuum pump.
@@joelsoncdma Yes - and its a fine line between high enough vacuum for a decent electron beam and and just enough ions for glow and show the beam. I have seen some people online make someone operational CRTs even with low vacuum from a simple pump so its doable. After your comment a few days ago I did some looking. There are acrylic bell jars available for vacuum classroom experiments. Fitted with a thick piece of acrylic at the base it might make a safe way to try making a crt.
I remember doing this way back in high school, 1975, on the football field with a sine modulated Metrologic HeNe laser (goal-post positions) with great results...don't recall the exact results though.
THANKS FOR THE GREAT VIDEO... fond memories...
Glad you liked the video and that it brought back some nice memories! You must have had a great since teacher in high school - I was never lucky enough to have done an experiment like that in high school. I was intrigued to hear of a HeNe laser being modulated. Just did some googling and it looks like you can modulated them to about 1MHz. I was surprised you can do it that fast one is dealing with increasing/decreasing ionization in a gas. I'll have to dig out my old HeNe laser sometime ans see if it still works ....
@@ElectromagneticVideos Thanks much for the pleasant reply,
The gas laser was actually 10% (IIRC) intensity modulated with an RF amplifier tied into the power supply circuitry (it could even transmit TV pictures as well as audio). Surprisingly, the transmissions were exceptionally clear! I think there are even some of those old METROLOGIC manufactured HeNe's on ebay (long white units). I enjoyed that project, but my real passion was holography. It took some time to get the homemade optical setup done, as well as getting the hang of developing them in my bathroom-converted darkroom. I'll never forget the thrill of being able to make out the reflected glints from my first attempted hologram...from a monopoly hotel roof. Later, the images were so real, that you just had to reach out into thin air attempting to touch the ghost image. Even now the smell of stopbath (acetic acid) brings back those good old memories (silly, I know).
BTW I really enjoyed that EM CHAIN! I'll be looking into your other videos, for sure.
can you post the part numbers of the laser diode and the photo diode, please
Here they are: Laser is PLT5 522EA_Q, Photodetector is PDB-C156. I bought them from Digikey. Mouser seems to have both as well. Careful with variants of the Laser - some are higher power. I would avoid them for the sake of eye safety. Also look into eye safety with lasers if you arnt familiar with the topic - the laser is considerably stronger than a laser pointer and could do damage if used the wrong way.
If you do the experiment, please let me know how it went!
@@ElectromagneticVideos Thanks. Why don't you use the rising edge of the laser diode excitation pulse as your time reference and then just use 114' for distance? I mean why do we necessarily need the mirror to create an extra shorter path? Does the laser diode have a significant delay in emitting the light pulse since it receives the electrical pulse from the function generator?
@@amirb715 If you look at the two scope traces in the video, in particular where the the 3' round trip pulse is shown, you can see the 3' round trip pulse starts rising at the beginning of the (upper trace) voltage to the laser, and start dropping when the laser voltage goes to zero. So there is time-constant-like ramp up time for the laser and too (I think) a lesser extent with the photo diode. So the easiest way to take all of this into account is to look at the position of the 3' round trip pulse on the scope, and use that position relative to the 100 foot round trip time to get the 100 foot delay.
well perhaps correcting the calculus with the coefficients of the 2 lenses, you will have better results, since the glass due refraction slows down the light. I was short but you understood what I'm refferring to 😊 what do you think?
To keep the math simple, you assume the thickness of a lens 0.25 inches and absolute worst case index of refraction of 2.
Time of flight though air of 0.25 inches = 1 ns/ft x 1/12 ft/inch x 0.25 inch = 0.02ns . With and index of refraction of 2, the time to get through the 0.25 inch is twice that = 0.04 ns.
So the extra time to get though the lens is 0.02 ns is so far below the ability to measure time in this setup (+/- 5ns?) it really makes no difference.
@@ElectromagneticVideos yes but at least is aken into consideration and verified its magnitude. However it's great and I can't wati for your next video 🙂
Also keep in mind you did this experiment in Air ( with atmospheric pressure ) and light velocity is slower in air medium so your result might be little closer to accurate.
The index of refraction of air is about 1.0003, so the slowing effect is around 0.03%. For my setup, way smaller effect than the sources of measurement error. But an important point for someone was setting up to try and get sub-percent type accuracy.
@@ElectromagneticVideos ooh
Does the response time of the photodetector come into play?
Regarding the response time of the photodetector: You are on the right trail asking if it matters. First notice that the top pulse has a more square-like shape, while the bottom trace has a rounded rise and then fall. Technically, this rounding is an exponential shape, and caused by a "time constant" set by the combination of the "100 ohm series resistor", used with the detector, and the "total capacitance." Here the total capacitance equals the capacitance of the detector, plus, the capacitance of the cabling to the oscilloscope, plus, the input capacitance of the oscilloscope. So using different oscilloscopes would change the rounding some. To get a more accurate measurement of light speed, ideally we want the return pulse to have a very sharp rise and fall, giving us a more exact measurement of the time-shift.
How to get a measured pulse with a sharper rise time and more accurate measurements?
Option 1) Use a "low capacitance" oscilloscope probe; generally these are "divide by 10" meaning they also reduce signal into the oscilloscope by factor of 10., Option 2) Buy a more expensive photodetector rated for lower capacitance and higher speed, 3) Lower the "AC impedance" of the existing resistor by putting an additional capacitor (that has its own resistor in series) in parallel with the existing resistor. From the video, the existing resistor at the detector is 100 ohms. For option 3, putting an additional capacitor value about 10 nanofarads with its own series resistor in its leg of 100 ohms should sharpen the rise time by a factor of 2. Important that the newly added capacitor and resistor are in series with each other, and this pair is across the existing resistor at the detector.
A bit technical, but measuring analog signals with an oscilloscope is a window into a level of reality that most never see. Takes time to understand, but pleasing to one with the patience.
@@thomaskoscica7266 I don't think it actually matters in this experimental design. The diode's time constant is, well, constant, due to the constant resistance and capacitance. It will take longer to charge based on the intensity of the incident light, but note that he's actually measuring the difference between the peaks. The peaks are formed when the diode switches from charging (i.e. getting struck by the laser) to discharging (i.e. between laser pulses). It doesn't actually matter how long the diode takes to charge or discharge, the peak will occur at the same offset relative to the laser pulse's removal, and undergo the same "rounding" process, for both the short and the long trip. All that really matters for this experiment is that the two peaks are far enough away from each other (temporally) that it's possible to take a measurement.
@mmaranta785 Two kind viewers already responded to your very valid question, so many thanks to them.
The quick answer is, no, because both pulses (from the short and long round trips) are detected by the same diode, so any response delay or lag, or shaping or smearing of the pulses will be the same for both. So the peaks of the pulses may be shifted or smeared, but the relative peak to peak time should still represent the correct difference in round trip times. Hope that helps!
@thomaskoscica7266 Thanks so much for that detailed response to @mmaranta785 . "a window into a level of reality that most never see" - how true!
@michaelcalvin42 Thanks so much for responding to @michaelpupeza6692. Your answer really nails it. "I don't think it actually matters in this experimental design" is exactly what I was going for!
You also can use a rolls of fiber glass cables with different length.
Yes! That would give the speed of light in a fiber which would be a bit less than air, bit would be very relevant these days with so much of our data going though so fiber!
This really brings science closer.
Thanks!!!!
It never ceases to boggle my mind how that huge number is actually very slow when you look at the galaxy scale, but also at the computer electronics scale: a 1ns clock pulse is about 30 cm or 1 foot.
At those scales you have to literally wait for the light to arrive
Yeah - its so out of our normal life experience of the scale of things.
Funny though, as you point out, a 1ns clock pulse is not untypical of computer electronics or comms today and when you work in those areas ns is just a common everyday time scale.
I wonder if a cheap $2 shop semiconductor laser will do the same job. This will be particularly useful to reproduce it in secondary school environments.
It should do but keep in mind that it would produce lower quality light which is a bit slower.
I think the main issue is that usually nicely packaged cheap lasers like that include some sort of driver circuitry which probably slows down the on/off response. If one could find one that could easily be hacked, it would be great! When I do the follow up DIY video I'll take your suggestion and ask if anyone can recommend a suitable device like that.
Just to be clear, the light itself wouldn't really be slower. but the response time may be making the rise and fall time of the light pulse slower.
Just a note that the speed of light in air is slightly lower (299,705 km/sec). Not really a factor here. I like the differential delay approach since it cancels out the delays in the laser and opto-detector. Nice job.
Thanks! I also like the just seeing the two pulses on the one scope trace which at least for me makes the delay more "real".
We used the Michelson interferometer to measure the speed of light when I was in physics. It is fairly accurate. Seems like we measured it in the lab at 2.99x10^8 m/s.
I'm intrigued! I thought the Michelson interferometer was to measure the minuscule difference in length (or speed of light) between the two legs. Is there some clever trick to using it to measure the speed of of light? You certainly got a great result!
@@ElectromagneticVideos In laser interferometry, a laser source with known frequency is used, and the wavelength of the laser is measured by determining the distance by which the stationary mirror needs to be moved to cause destructive interference. Light's speed is then calculated by multiplying the frequency and wavelength.
@@williamwalker39 Oh - ok! Thanks!
I posted in the comments above another way to measure the speed of light using radio waves. It is a very simple experiment involving 2 dipole antennas and transmitting a carrier between then and measuring the phase shift on an oscilloscope as a function of antenna separation. The results reveal the instantaneous nearfield and the light speed fairfield you guys are measuring. My results show that the speed of light is not constant in the nearfield and this has serious repercussion for Relativity theory and science in general, because so much of it today is based on it. The experiment also shows that information can be transmitted faster than light using nearfield electromagnetic fields (light).
This would be a great science demo for junior or high school class.... isn't the speed of light measured in a vacuum? How much would the atmosphere slow it down?
Interesting video...
Your right it slows down in air - but only about 0.03% so the difference is way less than the accuracy one would hope to achieve.
I do think it would be a very neat experiment for a high school science class. Hopefully some teachers who are good at inspiring their class will see this video!
The higher bandwidth of your scope and probes will increase accuracy. Also consider using a tip ground to shorten your ground path. The coiled wire thing that is about a cm long and slips onto the metal ring at the end of the probe.
By tip ground you are referring to what I would call a ground spring? That certainly might have helped. Biggest issue was noise and that might have reduce it a bit more.
Very nice experiment! Using a delta in distance is a great idea. Oscilloscopes is a fantastic "cheap" tool for realizing such experiments. The only thing I don't understand is the "utility" to use imperial measurements. Fist time I see that in scientific work. 99% of the planet use metric system ;)
Thanks! Its funny how scope are now "cheap" - in the old days they se so expensive they were almost out of reach for many.
The reason I use imperial here is I find the "1 foot per nanosecond" makes it really easy to intuitively associate distance with time when one is getting sense of what is going on. And as a result of 90% of the video audience being south of me (ie in the USA ) I also test to often mention imperial measurements in my videos since that's what they are most familiar with.
You could of made it a lot clearer so that uneducated people like me can see exactly what is going on, a schematic would be helpful too, I really appreciate that you took the time and effort to share your knowledge about light speed, please make another video, I would like to know about the speed of light, another point I would like to make is there a difference between sun light and laser light. Thanks for sharing what you know. Cheers from me 😷👍👍👍
I'm working on the DIY video, which includes a schematic. Its always a struggle as to how much to put in the video - too much and any viewers get bored. So I decided try a 2 part video this time so people like yourself who are interested enough to want the details can look at, and other can skip. Will be interesting to see how many look at part 2! And I do have plans sometime for a video about sun (white) light and laser light . Cheers to you too!
@@ElectromagneticVideos Thanks for your reply, me and many others really appreciate it, we will look forward to the next video's on this topic. Cheers from me 😷👍👍👍
@@magicbytes3835 An you know,. I really appreciate viewers like you taking the time to comment - nothing like feedback!
Was there really a need to do this by considering the two different paths (3' and 114')? Just the difference between the drive pulse and the arriving pulse (after traversing 114') would have given you the answer. Notably, the 3' signal is indistinguishable in time from the the driving pulse on the scope (within the overall time resolution of the PD, scope, and all electronics between) because 3' is 3ns in time (on scope). By doing the two-paths method, I would say more uncertainty has been introduced into the experimental result. To get better resolution, need to reduce drive pulse width, improve PD speed, and substantially increase path length.
I felt the two path on one trace method makes the "the light really is delayed" a bit more obvious. And yeah, the 3' really makes no difference given the measurement accuracy.
Hi, interesting comment, I just want to check my understanding. Why would decreasing the drive pulse width improve things? I thought more important is how fast the laser diode switches on or off. If there is a wide drive pulse, as long as the switch off time is only 1 or 2 ns, then you can compare the tail of the drive pulse with the tail of the received pulse and the width of the pulses is not important.
@@declankruppa8300 Your absolutely right - if you had a laser (or any slight source) that could be switched incredibly fast and a suitably fast receiver in many ways it would be a much nicer experiment.
But those speeds are not easy to do cheaply - I did find a $20 1.5Ghz photodiode that should have ns type resolution, but one would need a high speed amp near the diode and an expensive higher bandwidth scope (my 500 MHz digital scope was $400 used a few years ago),
So the by adjusting the drive pulse width to what turns out to be about 100ns in my setup, the combination of the rising and falling time constants of everything combined produces a somewhat triangle like pulse. While the distances used are such that pulses overlap, the peaks are separate and that makes the short and long round trip pulses evident. So shorter, nicely formed pulses are partly useful to make the experiment a bit more obvious to the viewer and the purpose here was so nicely show the speed of light rather than measure it exactly.
Hope tat helps!
@@declankruppa8300 Thanks for the comment. You are right in saying how fast the laser turns off is important. If you look at the scope traces shown in the video, the cusp of the PD signal corresponds to the beginning of the fall-off of the laser output. Likewise, where the PD signal begins to rise is where the laser begins to rise. So, there are two places to do the timing. The point about pulse width is that if you keep reducing the laser pulse width (but keep fluence), you would ultimately begin to ALSO improve the rise & fall times of the laser output, and if the PD can keep up with the improved time resolution of the laser, the overall time resolution can be improved. Think of a delta-function laser pulse and the response of the PD to the delta-function laser pulse as yielding the ultimate time resoltuion. As shown in the video, the PD reponse has a significant parasitic capicitance, and a delta-function-like laser pulse would result in an instant rise of the PD signal followed by a decay profile characteristic of the PD. Notably, time-resolved spectroscopy nearly always strives to have short, delta-function-like laser pulses to improve time resolution. Hope this explains better.
Great lesson! Thanks
Glad you liked it! It was fun to do!
Huge fan of your channel. Want to thank you for your exceptional content and answering our questions. I'm curious why you based the measurements on the time between the short and long distances, instead of the time between the source and destination current peaks. Intuition tells me because reception response is the same. So why bother with source current?
Thank you so much! Why use the difference between the two received peaks rather than the difference between the current pulse powering the laser and the received peak from the photodiode? Because I was unsure how much delay there was between the start of the current pulse sent to the laser and when it would start to emit light. And also the response time of the photodiode. Add to that the complication of does the laser response time change as it warms up after a operating for a while? So by comparing the two peaks from short and long light paths, all of those uncertain delays are applied equally to both long and short measurements and shifts their peaks by exactly the same amount. So the only thing we need to know is the difference in round trip paths and difference in peaks times. All the other unknowns cancel themselves out. Hope that makes sense!
@@ElectromagneticVideos I see now. The unknown delays are equal in both measurements, and therefore cancelled when we subtract one from another.
Apologies if this was already addressed in part 2, which I plan to watch soon. --Kevin
@@d46512 Exactly! Not sure of I mentioned it in part 2 or not - and I was happy to explain it here. Its actually not uncommon to do things that way - often way easier that trying to characterize some parts of a system with extreme accuracy.
Enjoy part 2! If you have any questions or thoughts, don't hesitate t comment and I will try and respond!
You’ve mentioned “round trip” several times. Is it possible outgoing and incoming light travels at different speeds? I don’t know why it would be different but also don’t know why it would be the same. Great video!!!
Veritasium has a video about this. Basically all speed of light measurements are round trip because of the difficulty (impossibility?) of synchronizing clocks without certain assumptions.
Thanks! @chrimony response to your questions is so nicely stated!
Your are absolutely right though - while one cant really measure the one way time, it would be very strange of they were different. And contrary to what few commenters have said, technology like GPS works because of our understanding of the speed of light, time and gravity, (Einsteins stuff) and if there were significant weird inconsistencies, well, we would probably have issues with those devices!
Isn't the speed of light ~300,000Km/s in a vacuum?
Wouldn't that mean the air in the room slows the light down and thus the analog scope may be closer?
Your right - it does slow it down - but only by about 0.03% or so. So the difference is well below the accuracy of this experimental setup!
Excellent setup 👍🏼. I believe there is a need for a very large room to remake this experiment. I wonder if the mirror could also be placed outside the house or would there be too much disturbance (natural light etc)? Thanks for sharing your knowledge!
Thanks so much! In terms of room size, it really depends on how accurately you can determine relative positions of the returning light pulses. In my setup, the position of each pulse is probably +/- 5ns, so the difference between the two pulses is probably about +/- 10ns. If we want a +/- 10% accuracy, we need 100ns round trip time = 100 feet of round trip or a 50 foot long room. Even a 25 foot room would work with a reduce accuracy to about +/- 20%. I was able to reflect the laser beam from one end of my basement to the other though a hallway to get the 50 feet.
With a cardboard box shielding the receiver photodiode, it would probably work fine outside even in daylight - focusing the laser beam with the lens onto the photodiode really makes it work well. Even with all the lights on in the basement to the point where I could hardly seen the green from the laser. there was no visible degradation to the received signal. However, the big problem doing it outside would be trying to line up the laser, mirror and receiver. You would would probably have to do that at dawn or dusk or even better, at night to make the laser light more visible.
One thing I had though of for a shorter room would be to have a mirror at both ends and bounce the light back and forth a few times to increase the distance traveled. Of course its a more complex setup and more alignment issues ....
@@ElectromagneticVideos The idea, having more than one mirror to reduce the room size is very helpful 👍🏼. Thank you very much, I really appreciate your work!
@@gerd3136 Thanks! Are you going to try it? It was fun to do. Over the week I will do a DIY video with the parts list, some suggestions etc.
@@ElectromagneticVideos Yes, I will try it in the near future. I already measured the speed of sound, which was much more easier to do. Looking forward to your next videos 😊.
@@gerd3136 Fabulous! When you do please let me know what your result was - hopefully with a bit more care a bit more accurate than mine. How did you do the speed of sound? Do you have it posted anywhere?
Great video! Thank You for sharing this. I think you actually underestimated the difference of time between peaks in the case of digital scope. I understand that this is very hard to do. But I think if you compare "tail" of leading peak, then you should compare "tail" of the following peak. Sorry for this, but I think this is more precise than it seems. If you want I could try to fit your data to some "model" function to make it "more scientific". I want to underline that you put Your results without "messaging the numbers" this is quite a bit of a problem in scientific community to "filter" or "denoise" the data. Thank You.
Thanks! It was hard to get the exact peak position, not only because of noise but also because of the wobbling second mirror I has holding in my hand because I ran out of tripods.
"try to fit your data to some "model" function" - yes! Since we know the shape of the pulse, doing least mean square fit with a model pulse to find its best estimated position can not only average out the noise but also produce sub pixel (or time sample) positional accuracy. Even simpler, a "center of mass" type calculation for pulse would improve things as well.
As far as presenting the data as is, I think I also mentioned how Galileo declared his results as inconclusive - a great example of reporting a result correctly. I'm a bit more optimistic than you about data being fudged in the scientific community - there have been a few notorious cases in the last few years but it generally gets caught when others get different results.
We can only ever measure the speed of reflected light. What is the speed of reflection? And what is reflection anyways? And is the speed of light independent of the direction of travel?
"what is reflection anyway" or "how does a mirror work" is a future video I have planned!
@@ElectromagneticVideos We could avoid the "reflection issue". Instead of using a mirror we could have similar laser generator sending a separate beam back, that is triggered by the incoming beam. Or a beam that is sine wave modulated and we measure the phase difference. It would be interesting to see if we get the same result as a mirror - but of course the accuracy is nowhere near enough for that.
Another idea: Have two sine wave modulated light sources at each end, same frequency, and then move the detector along the path and measure the distance between phase cancellation.
@@georgedereck6525 Remember in this experiment as done in the video, any possible delays from the mirror are taken in to account by having both short and long light paths include a mirror and taking the time difference between them.
@@georgedereck6525 A variant of what you are describing is to use microwaves and measure teh interference patterns directly (ie standing waves).
Well done!
Thanks!
This seems like a good way to calibrate the time base. C is by definition set to 300000km/s exactly. I think they changed the definition of the meter to match the cessium reference
Minor correction : c is defined as 299,792,458 m/s exactly (300000km/s is close enough for most things and is a more convenient number). And a second is from the Cesium reference.
So you would probably start with time from an atomic clock and then somehow use that determine a meter length (maybe with interference/standing waves?).
Whether more distance would improve your agreement depends upon the primary source of your error. If it is due to your oscilloscope's timing calibration, then more distance would not help at all. If it is due to uncertainty or error in the distance between the wall and the (near) mirror, then it would help a great deal. If it is due to uncertainty in reading the oscilloscope, then more distance would help. In fact, expanding the time scale would improve this measurement from ~10% to ~1%. Finally, the relevant distance is from the near mirror to the wall, to the detector MINUS the distance between the near mirror and the detector... 2x57' doesn't seem to be accurate.
"If it is due to uncertainty in reading the oscilloscope, then more distance would help." That's exactly it in this case. And mostly as a result of the noise making it a bit hard to see the exact peak position. Looking at the Digital scope I think I misjudged one of the peaks a bit - hindsight is 20/20!
Distance - maybe I wasn't too clear - I did subtract 3' from the 2x57' to account for the two different round trip times. The measurements are accurate to a bout a a foot (=1ns) . I didnt worry about any more accuracy since I wasnt expecting to be able to read time to anywhere close to a ns.
All great points you make - well worth looking at for anyone trying a version of this measurement!
I would have thought you would have been able to achieve better accuracy. Trace 1 is irrelevant [upper] because it is measuring the pulse to the laser and wasn't used anyway. The discrepancy is in the measured distances and possibly the response time of the photo diode. An alteration of the experiment that might help is to split the bean into two using matched response time photo diodes that have precision measured distances each one catching their respective beams of the split. Also ensure that the wires from each photo diode to the scope are of the same length but send them to two separate traces. That way you can align the traces to the same level and easily measure the time differences between the peaks.
Although I didn't mention it (I have learned to keep things as short as possible the UA-cam audience) I had trace 1 displayed as an easy way to see which pulse (long or short round trip) I was displaying. The downward edge corresponds to the short round trip pulse peak, at least to within a couple of ns.
The multiple photo diode approach would sure work. I will mention that option when I do the follow-up DIY video as another way of doing it. My intent here was to keep things as simple/obvious as possible so anyone could understand it and the two pulses on the same trace seemed like a good way to do that. Your absolutely right about getting better accuracy with a bit of work. Noise was a big issue and trying to read the pulse peaks accurately. Some amplification right next to the photo diode would probably have helped, but I wanted to keep things simple.
Yeah but we are never really measuring the speed of light are we? Not directly. Like for example I would measure a popsicle stick. We have only ever come up with a way to measure the round trip and divide by 2?
I would suggest using EME Earth-moon-Earth (bouncing radio waves off the moon) as a final step to increase the accuracy of speed of light measurement. After that it would be way too expensive for any hobbyist to buy a powerful enough laser to bounce off of the retro-reflector left on the moon by Apollo 15. Because of temporal smearing caused by the radio signal bouncing off the curved surface of a sphere the signal peak will be spread out. But should be accurate to less than 1 percent error from the actual speed of light.
One of the commenters somewhere below who is a ham mentioned he had actually done that. It would be a neat experiment to do sometime!
Nice cancellation of electronics delays this way!
Thanks! Yes - and (to me anyway) having both pulses on the same scope trace makes the delay a bit more apparent.
The definition of the meter is the distance light travels in 1/299,792,458 of a second in a vacuum. So an accurate measure of light travel time dictates the distance that light traveled in space, INDEPENDENT of your lab, which we both know is in motion in space. Light travels independently of the lab. The light travel TIME is a measure of DISTANCE that light traveled in space, not the distance that the light traveled in your lab which also traveled a distance in the same period of time.
Pause the video to view the pic. Notice that in the box the distance from the center of the box to the Z receiver is .5 light seconds in distance, but it takes the light .65 seconds to travel from the center of the box to the Z receiver. Also notice the distance from the center of the box to the X receiver is also .5 light seconds of distance in the box, but it takes 1.384930 seconds for light to reach the x receiver. The distances are the same in the box, and yet light takes 2 different times to reach the receivers. That means light speed in the box is different when measured in the different directions. That makes Einstein's second postulate of Special Relativity false, which makes his theory Bunk!
ua-cam.com/video/udPDkq9tT-s/v-deo.html
Is light linear
As in an additive sense? Yes - in almost all common situations. There are some materials that do exhibit nonlinear behavior that can be used in interesting ways: en.wikipedia.org/wiki/Nonlinear_optics
probably the short path should be 2x3 ?
I must have not been the clearest on that - your not the first person asking - its 3 feet round trip. The individual parts are more line 1 ft + 2 ft = 3 ft.
Thanks a lot sir.
Glad you liked this one too. Its one of my favourite experiments!
I'd like to see what happens if you try this in a Medium Like Water to see if the speed varies & therefore not a Constant!
It will be slower - it slows down in denser things like water which is why light bends when it enters water at an angle (future video). Hippolyte Fizeau who first measured the speed of light by sending a pulse of light did did a lot of follow up work measuring the speed of light in water and other materials back then when it was all unknown. Today the relative speeds is given and the index of refraction which for water is 1.33 (so speed of light in free space is 1.33 times faster than in water).
You can do the same experiment to see the speed of light (in that case a RF pulse) in a wire which is another case where it it slower. I did that here: ua-cam.com/video/a-8CaGOmWDU/v-deo.html
@@ElectromagneticVideos I was always skeptical of it being a constant & argued with my old Physics Teach that there's No Such Thing as a Closed System in Nature where a Vacuum is Completely Empty of All Matter & Therefore Using it as an Example isn't Valid.
@@Shivaho It actually get even more interesting than that - there are actually two speeds of light - the phase velocity (the speed the waves move) and the group velocity (the speed they carry information or energy). They are the same in free space, but different in many other situations ... also a future video!
@@ElectromagneticVideos Cool I was a Science Geek in High School but the Arrogance of My Teach turned me off to a career in it. Still I'm Fascinated by Space & Science but also find that the Missing Link to Everything is the Nature of Consciousness & it's Impact on Humans Perception of Reality. It's the One Thing Science Can't Seem to Wrap their Heads Around & is Why They Can't Truly Explain the True Nature of the Universe.
@@Shivaho Thats so too bad! I sometimes cant believe I still liked science after many really boring and uninspiring classes in school. Luckily I went on to do it in university. The great thing is all the wonderful science stuff on youtube. Have you see Periodic Videos (chemisty) and Numberphile (math) - some of my favorites!
1 what about probe wires error?
in probe wires electricity doesn't Travel at the speed of light.
2 or the scopes processing data.
to process speed of light data it needs to be much faster than the speed of light itself.
(magnetic field wich moves election rays on the scope can only move it slower than the speed of light)
3 reactance speed of photo diode and laser.
4 rise and fall speed of the pulse generator (thus also affected by the velocity of the probes)
the test is awesome.
I re-created it in my shack with same results.
The speed of signal in wires is typically a bit less than the speed of light. In most coax cables, its usually between 60% and 90% the speed of light. Since the probe cables are only 2 or 3 feet long there is only a 2 or 3ns cable delay. And since they are about the same length, and we are looking for the difference, the delay times cancel.
Scope - it isn't faster than the speed of light since the distance between the two peaks is perhaps 2 cm, way less than the total distance the light travels in the experiment. And even if it was more - the dot on the scope may appear to move to our eyes, but in reality is just where some electrons land. It could in theory move faster than the speed of light because the electrons dont follow the dot position.
The generator rise and fall and detector. All cancel, because we look at the outgoing and return pulses with the same detector and probes. So there are undoubtedly delays from those things, but again since its the difference that matters, they cancel.
You re-created it! How fantastic! Did you use the same laser and photodiode? Or did you use difference ones? What value for the speed of light did you get?
Speed of light in earth’s atmosphere is slower than in a vacuum. Also see veritasium’s video on why we can’t measure the speed of light.
super i think we must 1st synchronize the oscilloscope with a 10MHz GPSDO (Global Positioning Satelitte Disciplined Oscillator) then we have a better much exact result. Also the long way can be better in case of a more longer distance. But thank you for this cool Video!
A timebase error of a few parts per million would be insignificant compared to the response time of the photodetector.
As @stargazer7644 indicated, the scope timing issues were really a less significant problem. The biggest issue I found was noise making it hard to get the precise pulse positions.
So glad you liked the video! Thanks!
@@stargazer7644 the response time on the photodetector is from the direct beam and the indirect beam I think all the time the same. Maybe it depend on the falling in angle - that this make sense if you run it by a prism. The Analogue Oscilloscope are for sure 10% wrong and also the Digital Oscilloscope is wrong in timing. If you mentioned noise so this could be phasenoise from the Oscilloscope itself - maybe we can install a amplifier after the photodetector this will increase the rising edge then we reduce the noise to. This Generator I use to but the arbitrary signal what come out is also a inhabitable signal so the start point is not correct. Maybe a blue laser light will also make more energy in case of the higher frequency. I think that we can make it more exact if we change some thing - but this is not to understand as a critic! I like your Video very much it clear tell us what happened with the measurement. Thank you again for it!
@@weakbit633An oscilloscope is not ever likely to be 100,000 ppm (10%) out of spec. They are calibrated instruments. I work with them every day and I've never seen one out that far, even after decades without calibration. They're designed to measure signals and they wouldn't be of much use if they couldn't do that. The response time on the photodetector will depend on its capacitance which will ultimately limit its frequency response. Yes it will apply to both beams and add a fixed offset to the timing.
@@stargazer7644 But this is a Philips Oscilloscope this is minimum 10% out of spec (read the manual) and if it is old like this ~50years (this type is from the 70s) then it is totally out of spec! All Analog Oscilloscopes are have a measurement fail from about >10%. A analogue Oscilloscope is not a precision instrument more a estimate Instrument. This Oscilloscope is Service Oscilloscope and not for precision measurement. Also this Generator for €60,- is only a problem in all - this Generator is also not Synchronized and I've this generator is the cheapest instability arbitrary generator. - But I don't criticism the precision of this measurement equipment. You never can read a exact timing like the 120µs from this blur screen this is impossible. Also he mentioned that the noise is some problem. Hahaha calibrated! I know nobody that make a calibration on a service scope! This scope is near 0years old and nobody will make a calibration!
I find it a bit ironic that most of us are using almost the exact same technology to watch this in the first place, just through glass instead of air.
Ha ha - yes - how true!
Bravo, well done experiment! I noticed and interesting phenomena with with large Tesla Coils fres
Thanks!
"quarter wave resonant frequency based on the wire length" - what wire and how much % higher frequency? The self resonant frequency of any coil (including ones meant to be used as a tesla coils) is the result of the self capacitance of the windings (and of course the inductance). Those stray capacitance can be highly variable based on things like slight changes on winding spacing, wire insulation, coil form permittivity etc so I would not at all be surprised at significant deviations from the resonant frequencies that one might be expecting...
Hey, I measured the time it took to walk to the refrigerator and back with a beer! It was 37.17452 seconds. (That was the resolution of my counter.) :)
And we all know how critical having those last few digits are :) . Actually you sure point out how with digital meters people get fooled into a false sense of accuracy these days.
I have that same Koolertron signal generator!
Its funny, mine is "JDS" but I have seen them with the Koolertron branding. I wonder if one Chinese company is now copying the others stuff, or just rebranding it? Either way, you cant beat what that signal generator can do for the price! May not be as perfect as an HP, but for a bit over a hundred $, wow.
Kind Sir, my "metric to mile" tool, says it's 322,000 km per second. Is that right? For I was taught many yrs ago, that light is 186,000 miles per second. Where did I go wrong kind Sir? I don't use metric yet! LOL.
186000 miles/sec x 1.609 km/mile =299,274 km/sec which is pretty close to the exact value of 299792 km/s. maybe the tool is wrong?
The speed of light isn't an "accepted value". It is defined since 1983 to be exactly 299,792,458 meters per second. It is kind of odd that you started out in imperial measurements and switched to metric half way through. An interesting comparison would have been if you did the same thing with 114 feet of coax or twisted pair to show the speed of light in wire is much slower than the speed of light in air. That would have been a good way to mention the refractive index of air and the fact that the speed of light in air is slightly less than in a vacuum.
You said, " It is defined since 1983 to be exactly 299,792,458 meters per second."
Yes, it is DEFINED by the definition of the meter. Since 1/299,792,458 of a second of light travel time in vacuum is the definition of 1 meter.
599/299,792,458 of a second of light travel time is 599 meters.
3,876,564/299,792,458 of a second of light travel time is 3,876,564 meters.
and what do you know...299,792,458/299,792,458 of a second (1 second) is 299,792,458 meters. Since the distance was 299,792,458 meters and the time was 1 second, it is a DEFINED FACT that the speed of light is 299,792,458 meters per second due to the definition of the meter. There is NO POSSIBILITY of the speed of light being any different than 299,792,458 m/s because it is defined. It can't be slower or faster.
@@rmeyers2021Actually, you have that exactly backwards. The length of the meter is defined by the length of the second and the speed of light. It is the distance that light travels in 1/299792458th of a second. They adjusted the length of the meter to fit the defined speed of light in 1983. And yes, the speed of light is defined, hence my use of the word "exactly" and the entire point of my comment.
@@stargazer7644 There is no "length of the second." A second is a unit of measure of TIME. A second is a specific duration of time. The "speed of light" is a SPEED, which is distance/time. There is no "speed of light" until you measure the time of light travel, which then defines the DISTANCE in the unit of measure of "meter".
There is simply distance and time. In order to have a unit of distance and measure distance you have to define the unit of measure, which the unit "meter" is defined by the TIME of light travel.
As you will notice in the pic in the linked video, the "speed of light" in the box is different in different directions in the box. In the box the distance is the same from center to each of the receivers, but light takes two different TIMES to reach each of the Z and X receivers emitted from the center of the box.
Nice try, though. ;)
ua-cam.com/video/udPDkq9tT-s/v-deo.html
@@rmeyers2021 FFS man. The second is defined as the time it takes for 9,192,631,770 transitions of the unperturbed ground-state hyperfine transition of the caesium 133 atom. The length of the meter is now tied to this definition. The speed of light is not a measured quantity, it is a defined quantity and is invariant with direction or motion. The length of the meter is not a measured quantity, it is also defined. This has been true since 1983. Nobody likes an asshat.
@@stargazer7644 So how do you know the distance of a meter if you don't measure the light travel time of 1/299,792,458 of a second?
Can you explain why it takes .65 seconds for light to travel from the center of the box to the Z receiver, but it takes 1.384930 seconds for that light to reach the X receiver??
I can't wait!!
Also nice to check if an electric signal travels by the speed of light
Actually, see this video for what amounts to the same thing in a wire: ua-cam.com/video/a-8CaGOmWDU/v-deo.html
@@ElectromagneticVideos just saw it, thx! Nice vid.
@@TheDutchGuyOnYT Thanks!
Cool!!
Thanks!
We can only measure a two way speed though
Exactly!
Bravo.........home work.......how far in Pico seconds.......in feet......cheers
Since you brought up pico seconds, was just looking ta the manual for one the laser measurers - seems to mention a 600ps pulse - if true thats quite impressive.
Only works if you assume that light travels at the same speed in different directions :)
Absolutely!