I think 1 thing is missing here: the actual speed of light in the usual sense is not the phase velocity, but the group velocity at the front of the wave packet (which determines when the first part of the wave packet hits something). The light wave didn't exist for all time, but the 1st part of the light wave reach the electron at some time, and only after that the electron can also oscillate. So even in the case of x-rays, the group velocity would still be the vacuum speed of light, but the refraction is determined by the phase velocity, which is why x-rays are refracted away from the vertical.
Thank you! I was just about to make a comment asking what the difference between "apparently" moving faster, and actually moving faster is, since he showed the front of the wave actually being pushed forward past the front of the wave in a vacuum, and that didn't make sense to me. Although he did make sense of some other things, this was the missing puzzle piece.
@@alexjohnward The Group velocity of light will slow down in glass and water. X rays and Visible light will slow down the same amount, but their Phase velocity and the angle of refraction will differ based on wavelength.
@@alexjohnward Nevermind, I've been reading more about Optics, and the Group velocity is dependent on wavelength. keep in mind that dispersion is talking about wave packets spreading out, like the envelope is getting broader and may be moving slower than the Phase velocity of the oscillating waves inside. (Wikipedia, Dispersion) "If a light pulse is propagated through a material with positive group-velocity dispersion, then the shorter-wavelength components travel slower than the longer-wavelength components. The pulse therefore becomes positively chirped, or up-chirped, increasing in frequency with time" So an x ray should move slower than longer Wavelengths. (RP-photonics, Zero Dispersion Wavelength) "Many materials have only one zero dispersion wavelength within the transparency region, with normal dispersion below that wavelength and anomalous dispersion for longer wavelengths. The group velocity then has its maximum at the zero dispersion wavelength. There, a light pulse travels with highest speed. For example, fused silica has its zero dispersion wavelength at 1.27 μm. For other optical glasses, far shorter values are common (often in the visible range), and more than one zero dispersion wavelength can occur."
Wow!!..blown away ...thankx mahesh...u r the best educator ever i seen on youtube...no one explained that way,they spoon feeded on this platform about light
That's one of the best videos on the internet! I really appreciate your work, because we don't have this kind of depth in high school's physics (speaking from Brazil here), so it's really great to understand it properly!
This is the absolute best and most rigorous explanation to be found anywhere on the web, or in any physics classroom (and I have a PhD in physics). Fantastic presentation.
After 3 years I finally got my answer, I have been asking this question to my teachers for ages but they just said ' THE LIGHT SLOWS DOWN 🤓' and if I asked why then some of them said ' LIGHT COLLIDES WITH GLASS'S ATOM AND LOSES SPEED 🤓 ' theni get frustrated and asked them WHAT ABOUT EINSTEIN, HE SAID THAT SPEED OF LIGHT (ie C ) IS CONSTANT IN ALL SITUATIONS , then they just ignored me.😅 Thanks Mahesh bhaiya ( Btw I'm still in school 😢 )
You sir, are the utmost representative of intuition and simplicity, just like Feynman. I'm pretty sure if you were to be involved in research some 30 years later (teaching higher students), humanity would be thankful to your contributions! :)
After watching your video its my time to showoff my knowledge in physics to my friends....i am sure that i blown their minds exactly like what you did to me 🤯🚶🚶💯
Great discussion. When you asked how the light affects the electron, my first thought was it should be shifted by 90 degree. The electrostatic force of the light wave on the electron is maximum when the wave is at a peak/ valley. But that means the electron is 'accelerating' maximum at that moment. With simple harmonic oscillators, the acceleration is max when it pauses and reverses, i.e. when it momentarily stops. And the 'wave' formed when it's not moving is zero and it would be maximum when the electron is moving fastest (just as the original electrostatic force is crossing zero). That would make the wave formed by the oscillating electron, 90 degrees out of phase with the wave that forces the oscillation. Of course, electron orbitals are probably much more complex than simple harmonic motion, but something at least to think about.
Wow. I found your videos just recently. I have an engineering degree from a long time ago and I wanted to understand special relativity. I have some textbooks. I’ve watched a lot of different videos. Your videos are outstanding. Thank you so much!
I just wanted to take some time to point out how useful your videos have been not just to my to my knowledge of physics but also my perspective on learning. It is easy to believe a subject or topic is boring and not worthy of learning just because of who is presenting the material, without taking into account that everything can be made interesting with passion and love for something. Thank you @floatheadphysics and I hope you continue to make videos
Best and fastest and most intuitive.explanation I've found on internet about the ligth transmission through a material. Before your explanation I had seen unclear explanations from some other well known physics youtubers, and I was a little clueless. Thanks for your clarity and simplicity. I really appreciate it, since it's not easy.....
I truly appreciate your content that you put on youtube,,,you are one of the best teacher delivering quality content,,,please do post more videos including dispersion of light ,i would love to have it... Thank you
Very good. The video from Fermilab got me halfway there but I was still unsatisfied because he didn't go into how the delay of the phase of the forced wave can vary depending on frequency. The x-ray thing too was surprising. I am now satisfied to the extent that I want - I don't feel like slogging through papers because I'm way too lazy for that! Thank you much!
bro physics with you is art ! No video explained why light is delayed in a convincing way except this one, you made the difference by going into the details with is great, the details really make ALL THE DIFFERENCE, it's like to show someone how a home was built brick by brick instead of showing him how the home was built floor by floor, the brick by brick explanation method is the best and actually you are the only one I found explaining why the light slows "brick by brick" ( with great details). Keep going like this bro you are doing awesome work, and know that it's DETAILS that always make the difference in explanation ( explaining step by step without jumping steps). BTW I also think the same way as you with intuition, I found myself asking to me the same question you asked to yourself and you thanked the same way as I tried to do !
@19:20 you might not know it, but this point here is the most critical piece of the puzzle by far. Light is *_not_* a wave, the EM theory is a "hydrodynamics" of photons, applicable when you have a lot of photons. If you try to follow a single incident photon (with a gedanken Heisenberg defeating microscope!) it may not even emerge, it could just get absorbed, or Compton scatter, or small chance for whatever other scattering cross sections. Your wave effect explanation really is beautiful, but some minor inconsistencies some listeners might note could be resolved by knowing that it is only a statistical mechanical explanation. EM fields are fictions. Albeit very useful.
The quantum aspects are totally different to the classical EM wave. These are not at all the same sort of wave. In fact the quantum wave function does not even "live" in physical space. It is not an E and B field.
I love how this topic has been covered by you, 3blue1brown, and Looking Glass Universe, and you each have different ways of approaching how to explain this complex phenomena. Thanks for doing this work and bringing such passion to every video!
This is a brilliant explanation of something I had never even considered. It's almost 60 years ago since I was in college studying Microwave theory, which covered a lot of the same ground - wave phases adding or subtracting - but it was on a space commuications course (NASA), and we didn't go into optics. I knew from general interest how light refracts in glass, and I had always accepted on faith that it is because it slows down in media denser than free space, but the idea that it doesn't slow down - it just takes longer to traverse a glass block of fixed dimensions - is a relativistic thing - and Relativity is not intuitive. But I didn't know that X-rays refract away from the normal. I can't say that it is all plain and clear to me now, but I can, at least, follow the argument. So refraction in glass, then, is dependent on the wavelength of the wave being refracted. I can't see if it would be directly proportional (I suspect not), but I'm sure there is a fairly simple mathematical expression that relates wavelength to angle of refraction. I've just subscribed and become the 565th person to give it the old thumbs-up, and I'll be looking for your other videos. Clear explanations of scientific concepts are few and far apart! I've only seen this one video of yours, but I put you in the same bracket as Feinman - and, incidentally, Eintein, whose book "Relativity" (ISBN 0-517-02530-2) is also a brilliantly simple, non-mathematical explanation of a subject so complex that when he first proposed his ideas on Special Relativity, only a few other scientists of his day could come to grips with it. I'd like to ask some questions, if anybody can answer them ... 1) Are there transparent materials other than glass where the natural frequency of their molecules can be determined (eg water, or perhaps dense gasses), that obey the same refraction formula, based on frequency of the incoming wave, and their own natural molecular frequency? 2) Has the time for light to traverse a block of glass been experimentally determined? Does it back up this lecture mathematically?
Hello sir, I got truly fascinated after reading your comment. Like how the majority of comments here are of Indian high schoolers [ 10th grade, which I am in also now] so this is my attempt to solve the questions you asked. 1. The thing is that glass since glass in it's latent form has excess electrons [ which can be recalled from the rubbing of silk and glass rod ] so in the video, he does not clearly show the happenings for the formation of another wave outside of the light wave because I think that it is very complex to understand that part so for the general audience it has been kept as unnecessary headache to teach. But the thing about glass is that it is uniform, so the late phases happen uniformly which leads to rarer to denser refraction. But in the case of say water or dense gas, then yes the laws of refraction says that the incident ray and the refracted ray all lie in the same plane, so the refraction formula is already there in the form of snells law, or refractive index according to the speed of light in vacuum. Those things are in my own school textbook so it will be easier to explain the formula in regards to snells law since our curriculum does check the basics and easy to understand concepts only for 10th grade level since we have social science, biology, mathematics(mainly geometry, introduction to trigonometry and statistics) which just covers almost everything basic of our high school. Then in grade 11th and 12th[ Indian system of classes] we have choices of medical or engineering in the science field, where in the pre engineering classes we will be taught calculus, advanced trigonometry and other things In mathematics, while in physics and chemistry it is thermodynamics, kinematics, optics, electricity, atomic physics and much more, and in chemistry we are taught about the atomic structure, many laws, physical chemistry, and organic and inorganic chemistry to prepare us for the college courses. So to sum it all up, I still couldn't understand the topics very well that have been taught in this video, cuz physics and mathematics requires a hierarchical system of understanding, cuz without the basics, you can't understand the system presented by mathematics and physics. So I would like to learn all of this in detail later on in my higher studies but that is all I can answer for now
@@VictorKashyap-ie3zq Thanks very much for your considered answer. Most answers I get are two lines long, or simply "lol" or some other thought-provoking wisdom of the sort. There is another video that shows the wave interference in a better way, I thought, but his conjecture is that light is, indeed, slowed down in glass. Still, if you have time, I think the wave illustrations are good, and show much the same information as in this video. ua-cam.com/video/CUjt36SD3h8/v-deo.html I saw another video that claimed that light could be slowed down to 38MPH! Look up "How to slow light" I can't remember where I found it, but I don't think it was on UA-cam. Something that occurred to me about *_this_* video is that at 2:52, he talks about light going through a thin piece of glass, and he talks about the phase change of the wave that causes the light to refract, and the explanation "hangs together" - except that it moves from that virtual thin piece of glass into the next virtual thin piece of glass, but if it bends as he says, you would expect it to bend in each virtual thin piece of glass, so that it would go through the glass body in a curve, eventually getting lost in a spiral. Well, we know that doesn't happen, and I'm sure the lecturer (FloatHeadPhysics) does too, but whatever happens only happens in the first virtual thin piece of glass, where the light enters from a different medium (air, usually). It is dependent on the _interface_ between air and glass - not on what happens in the glass itself. However, I do agree that if X-rays are refracted *_away from_* the normal, then some other explanation than the speed of light has to be sought. I have asked a question on the other video about that. To be honest with you, though, I don't think I'm equipped to argue this case. I learned microwave theory and semiconductor theory (atomic structure of insulators and doping to produce semiconductors) a very long time back (I'll be 79 soon), and I've partly forgotten, and atomic theory has moved on a bit also. Now, for example, they are starting to doubt the structure of atoms - they aren't a nucleus with electrons flying around them any more - everything is *_fields._* I dunno - but then - I don't have to. In 5 years, I'll probably be gone - and I'm OK with that. I wish you well for your continued studies. I'm sure you'll learn much more than I ever did. As in the song by Louis Armstrong ... "I hear babies cry I watch them grow They'll learn much more than I'll ever know And I think to myself What a wonderful world!"
Just a thought - could this phenomenon (refraction, and the bending ot the beam) be illustrated in any way with a compression wave like sound? It would be a lot easier to study, and to visualise.
@@DownhillAllTheWay hello sir, tommorow I have my mathematics exam, so tommorow when I will get time then I will do my studies on it, because my syllabus has already been completed, so getting the chance to answer and research these has been feeling great and improving the knowledge. The song you had written has given me an awkward feeling, not of remorse or sad or happy or grateful. Just an unexplainable feeling of talking to a person who is not scared of his death, or that me being a young guy has not experienced life so for me I will welcome death but after when I fulfill my desired time of life, then I will not regret much, just like you
There's a little problem in 16:40 time mark. It shows like the X-ray passing through the glass can reach the right side of the screen sooner than the X-ray passing through air. A more reasonable explanation is that the X-ray phase is delayed by more than a half wave per wavelength of propagation, creating an illusion that it was sped up less than a half wave. Hence, actually the X-ray propagates through glass even slower than ultraviolet light, which is slower than visible light.
Let's say that the wavelength of the X-ray is 10 Angstrom, and the atomic spacing of the glass is 1 Angstrom. The phase of the X-ray is delayed by 7 Angstrom after penetrating the glass as far as 10 Angstrom. It creates an illusion that the phase is sped up by 30%. In average, each atomic layer of the glass delays the phase by 0.7 Angstrom.
I completely agree. Look how he gets there. (around @15:00) He says "there is another way energy can be conserved, that is if the blue wave is ahead of the yellow one" This is of course not physically possible. The effect cannot precede the cause.
I also don't think he uses energy conservation as an argument correctly. He first says the amplitude of the yellow wave remains constant because he assumes the glass has no absorption. And then uses energy conservation to argue the blue wave cannot be pi or 2pi phase delayed. Where does the blue wave gets its energy from?
I have struggled with these concepts for years . Thanks to your explanations, physics has now become so much more interesting, and easy to understand. I have watched so many videos trying to understand quantum physics but believe me you are the best . Keep up the good work.
Your understanding of physics is in another level From my school life i was curious and also tried to feel , understand what is happening and how.. Bt due to proper resources i can't think like how i want.. Then i shift from physic department to engineering bt i cant forget physics from my heart However u r a genius person like me😁😁
I would say that the more intuitive way of thinking of the xray situation isn't that it pulls the wave forward, but rather it seems to because it delays it beyond a half wavelength so it appears to move the peak forward through a kind of wagon wheel effect
i used to think that photons had to move through a more zigzag way due to more congestion of particles which is greater in length compared to a linear distance,and then we were to consider its velocity through a denser medium as the ratio of its linear displacement to elapsed time which will obviously be smaller.Watching this video helped me think how wave nature of light can be fitted the same way,thanks!
I always had this doubt in my mind that why is it said that light slows down or sppeds up because according to the Special Theory of Relativity, the speed of light is a constant, irrespective of the medium it travels through and then I would think what happens during refraction. I even asked my teachers but didn't got an answer. But now I know the answer just because of you! I really thank you soo much for this video and all the other amazing video that you have uploaded (also on Khan Academy) I just had one question, why does the light bend? The light wave just got shifted backwards due to the resultant new wave but why does it bend away or towards the normal?
"the Special Theory of Relativity, the speed of light is a constant, irrespective of the medium it travels through" Einstein never said that and it is not in the Special Theory of Relativity.
Not sure that the X-ray wave is pushed forward, that would be the wave influencing the electron before it gets there. Tather, the X-ray wave is so small that the influencing wave is pulled back beyond the crest of the wave, resulting in the phase of the previous wave appearing in the next wave. (if that made sense)
I still don't understand how this resulting phase change of the light causes it to bend 🤔 Also, shouldn't the electric field of the electron also affect light behind it once it starts oscillating?
Agreed! And the "energy conservation dictates phase change" description ignores the energy transferred to the electrons to get them oscillating kinetically in the first place. And if materials can be considered as being essentially a vacuum in this (glass) case, why not in all cases - why don't all non-metallic amorphous materials transmit visible light then? Mahesh is brilliant but this video raised more questions than it answered.
I'm with you. My *guess*: remember the light bend towards normal only happens when the light wave is at an angle to the glass -- if the light enters the glass perfectly perpendicular to the glass, it propagates through the glass with no angle change. The higher the angle of the light entering, the more the turn towards normal -- and I suspect the difference in the number of atoms on each side creates the imbalance that results in the angle change. If you consider the lightwave just a fraction of a moment after it enters the glass at an angle, there's just one glass atom above it that starts oscillating, whereas it's already got many atoms below it that are oscillating, which if you did the vector math, the imbalance between the induced waves of the one atom above and the zillions of atoms below, causes both the phase change and angle change. That's why light entering at a more oblique angle sees a greater bend towards normal, less oblique sees less change, and with a perfectly perpendicular beam feels perfectly symmetric induced waves on either side (so it gets a phase change but no angle change)
@joet9267 i do think one of the things that was missing from his explanation, in order to simplify it, was that he is only showing one propagating wave but in reality there would be lots and what we see is the probabilities of all them and all their interactions combined which is what causes the change in angle. I say this because otherwise you would not see the same phase shift when light entered perpendicular to the surface of the glass. so the phase shift and the angle traveled would have to have different causes if it happend at all angles. But as i write this i realize that still would not explain why x-rays bend the opposite as light waves based on the imbalance of oscillation idea joet9277's idea and i'm still not convinced i understand this whole phonominum. What am i still missing or misunderstanding? anyone have further thoughts?
also, even non transparent materials are mainly empty space so why do they reflex light instead of letting light pass through mostly unimpeded. I would suspect it's more about the field interactions, which are way bigger than the matter itself, and orientations of the molecules themselves. I would love further help to understand this whole topic. i have clearly missed something or there is still missing information.
I love your video! Thank you for this work! However, it opens new questions: 1) If the wavelength in glass becomes shorter then it should become shorter and shorter for thicker glass. And as far as I know, this is not what happens in reality. so, what is going on? 2) I was taught that photons deliver their energy to electrons only in packest of energies, shifting the electrons into different energy levels around the nuclide. So, how can the photos deliver just a bit amount of energy to the electrons? Thanks again
2) You're mixing up two things. The quantization ("portions") of electron energy is a concept for the "orbit" the electron is in. We don't change the orbit here, we just slightly "wiggle" the electron on it's orbit by it "feeling" the electromagnetic field that is the photon.
I am amazed that it still travels at C. You didn’t emphasize that this also caused the change in direction. I am a bit on clear how the pulling or pushing changes the direction. I really understand better with your explanations.
To have spectrum from prism, light is bent due to inelastic collision, massless of light becomes mass particle as you can see the light on the surface. Re-absorption of energy happens at the same moment for light to bend in order to achieve light speed, visible least energy of absorption is red light energy due to least bending, gradually visible higher energy of light absorbed due to bigger bending, such as violet, that is how the rainbow is formed. There is no light interference in double slit experiment, the formation of pattern is due to multiple source of light rays after through the diffraction grating plate, you can try by making 2 rays from same torch light and shine towards double slit plate, and cover one of the rays for your verification . There is light diffusion and there is no light diffraction. By shining light to a bead., you can achieve centered spot of image for small round bead, but you will get off centered image spot for larger bead. microwave and X ray are transmitted by alternating the current, the alternating current of x ray is faster than microwave. Imagine a small ball, you hit with 2 hands, continuously by applying same force, you hit the ball with open arm by imagine it is microwave, and you hit the ball with short distance gap by imagine it is X-ray. The energy apply to X ray is higher in this case. that is the reason why microwave can heat up matter due to hitting atoms up and down with each other, whereas X-ray is able to penetrate matter deeply, the atoms in the lattice experience very quick vibration without atom hitting each other and remain in the lattice as they are, then clear X-ray photographs is achievable. Light does not travel through wavelength or frequency due to massless. light travels at light speed regardless of medium or type of lights.
Interesting point of view. I'll need to think about it more, I'm not on board w/it, especially the xray theory but understand the thought. In my mind with xrays being much higher freq, the electron will only be 180 out of phase every so often, not for each wave. The video post; The Primer Fields Part 3 may interest you, maybe check out. The xray reaction has issues in the Primer Fields too. Excellent subject, look forward to having some extra time to think about it. Nice post. Look forward to reading the comments.
Mahesh... loving your channel. Would love to see you give your intuitive explanation approach to the similar phenomena of RF in various antennas. Thinking Yagi Udas, Cassegrain reflectors, phased arrays, long wires, and small transmitting loops. A discussion of directivity and impedance matching would be amazing.
Back when I looked at the phenomenon of light of different colors being diffracted as it entered mediums that were optically more or less dense , it always annoyed me that the textbook answer , more or less , was that the optical light is " slowed down " as it enters , say from air into glass . I was annoyed because I knew that light cannot " speed up ", or " slow down " in a medium - it must always have the speed c ( c= speed of light ) relative to the observer . Your explanation goes a long way toward clearing that up for me . And very interesting as well .
So, what I'm understanding is that refraction is redshifting/blueshifting the light; by changing the wave's length, it's changing the ...'type' of light. So in the example of X-rays, it would be shifting the light inside more towards gamma rays, yes? Redshifting/blueshifting of light occurs for the preservation of c... but if refraction needs the oscillation of electrons for refraction of a light wave... what causes a counter-wave/wavelength change necessary for redshifting/blueshifting within a vaccuum as the distance between distant celestial bodies increase/decrease?
Than you for this cool video. It blew my mind also as you showed that with the conservation of energy you can guess the position of the secondary wave. AMAZING :) But here 2 additional informations: 1. As the electron moves up and down it emitts the Photon with a phase of 90° later according to the displacement. So the blue wave (secondary wave) is in the minimum 90° behind the yellow wave (primary wave) up to 270° behind the yellow wave. 2. Optics books never explain the situation, what is happening during the time as the first wave hits the glas. It always describes a steady-state situation: The light wave already passed through the glas, as we see it here in you explanation. So the claim of beeing faster then light ist mathematicaly correct, but the information actually already passed through the glas cause the whole glas is already filled with a light wave. So no information is moving faster then light. Never saw a mathematical explanation what is actually happening as the first wave hits the glas.
consider the analogy of exciting one tuning fork with another tuning fork of a different natural frequency. Energy conservation constraints will require the excited tuning fork to ring at the same frequency of the exciting fork, but somewhat "quadrature" in phase. Amplitude of the ringing of the excited tuning fork will depend on the difference between the natural frequencies of the two forks.
Would wish you would use a pop-filter in front of your mic. The popping sound can become distracting especially listening with headphones. Very interesting videos. Love them.
21:07 can we just say that the electron in the mean position is in Direct influence of that electric field and the electrons above and below will get to the direct influence of electric field but after some tiny time delay.... 🤔
@@Mahesh_Shenoy i understand.... That's the main reason i said i am mostly wrong... But we can perhaps say that the intensity of the electric field is more at the mean position than other positions which causes different acceleration to the electrons in different positions which may results in the case above in the video.... Probably 😁🙄 I literally studied only high school physics and currently in that phase preparing for JEE Which is in next 23 days..... And yes your channel helped me amazingly a lot in visualising the physics.
My own, admittedly, imperfect analogy is to think of crossing a chasm on a bridge made of swinging boards. The boards' swing frequency can not be changed and the runner's step frequency can't either. However, long legged runners swing their legs slowly and short legged runners swing their legs fast. So all runners have the same speed (lambda times f = c). A runner who swings his legs slowly may find himself hitting the boards when they are going back and so is always delayed. A fast swinging runner may hit the boards when they are swinging forward and, thus, crosses the chasm faster. It all depends on matching the leg swing frequency to the board swing frequency. If a runner's leg swing just matches the board swings he may end up not feeling the boards at all and his timeing to cross the bridge would be the same as with non-swinging boards (refractivity index of that bridge would be 1 for his frequency). If the board swings are just as fast as the leg swings, the runner may end up going nowhere and it could be said that he has been absorbed; as in red light not passing through blue glass. There's a reason why swings are always used to explain resonance frequency.
Ohh Wow What a great explanation ever Thank U thank u thank u very much I am very very happy now. I am having a confusion on it ❤❤❤😁😁 keep going.. 🙌👍👍👍
It makes sense how the light gets pulled back but why does it bend? You should something like light is going forward but an effect of coming back shows. Like when a wheel moves fast forward and it looks like its moving back or in a fan sometimes or a broken tv screen. Sorry it might be dumb question but wanted to learn more
It bends due to Huygens principle, look up Snells law through huygens principle and you will see that it all depends on the angle of incidence and the lights speed in that medium, this video explains the latter
Ιt's like pushing a ruler into water at a constant speed but at the same time the ruler lenght inside the water shrinks! Similar to the wavelength of the light that shortens inside water compared to air. Therefore, although the speed of light (photons) does not change and is always c between the vacuum inter-molecular space of any medium because the ruler length shortens as it goes inside the water it will take more time to reach the other end of the water medium and exit. So effectively the speed of light goes slower in water υ=λf with f frequency being fixed and wavelength λ getting shorter in water therefore effectively the speed of light inside water υ is slower.
I didn't understand the part you said the electron and light wave wouldn't be in the same phase because of energy conservation. Do the electrons say I better not be in the same phase because i don't want to break the law of conservation? What's the mechanism there? I am a little confused.
When passing light, does the temperature of the glass remain completely unchanged or does it heat up? If the glass shows an increase in temperature, wouldn't that indicate absorbation and dissaption of energy from the light source?
Hi Mahesh, your channel is my favourite for understanding physics from ground up, intuitively. You are awesome at what you do. I have a small stupid question regarding this explanation and I would be obliged if you do take time out to address my query. You say that an infinitesimal strip of glass does its bit to 'pull a wave back', so the more number of such strips that we consider, more is the 'pull back'. So, the 'apparent velocity'/'perceived velocity' of visible light decreases with the increase in the number of such infinitesimal strips. In other words, the apparent velocity must somehow depend on the width of the glass cuboid. But we know from high school physics that the apparent velocity inside a medium depends only on the medium and not its geometry. Is there a fallacy in my understanding because I am unable to reconcile these two?
You could have put diagonal electron waves in the picture to show how the lagging works or draw some kind of triangle to show the length difference. But that was the only part I had to skip to because I didn't understand it immediately. And the fact that I understood the rest what you said very well shows that nonetheless your explanation was really good. Great video👍
Your explanation is amazing. But I still have one question, it might be silly though. When you demonstrated light passing through the glass layer made of 1 electron, it made sense. But when there are multiple layers of glass, thats when it got confusing for me. Because if we have multiple disjoint layers of glass, the light wave would show multiple refractions. But if we have a big glass cube, it will only have a single refraction. But according to your explanation, a big glass cube should also show multiple refractions since it would have electrons one after another just like disjoint slits of electrons as shown in the video. So why do we see refraction only happening once in a big glass cube instead of multiple small refractions as shown in the video? sorry for my bad english but this question is confusing me a lot. And thanks for the amazing educational content!
Sir i think this was all about the speed of wave which you explained beautifully and someone has a good knowledge of interference can understand it easily but you did not explained why it bends? Or did I missed something 👀
I would be interested to hear if you have any similar explanation of the bending of light seen in gravitational lensing? Also why certain materials are transparent and others are opaque to light and why some reflect it.
I have an observation and a question/request for an answer. I take it at the right UV frequency there is no refraction? I would like to see how the light bends back at the end of the glass if that is a vacuum? And intuitively it makes sense to believe that the wave would carry on the same as it did before in the glass as there are no electrons for to interfere with in the vacuum but that is not the case as it bends back to the original path? Intuitively it seems more like the beam is getting nudged to one side but still attempting to travel the same direction hence it comes out displaced but still pointing the same angle as it entered the glass?
❤- Thank you Mukesh - Would you consider making a series on gravitational lensing? - Eddington's photographs of the 1919 solar eclipse and what causes the observed deflection of the light source from distant stars by the Sun. What does Fyman say about this matter?
The actual bending, not the slowing, is result of phase and vectors acting on the wave. Though the light can approach at one angle, the "shape" of it at any moment makes it more or less susceptible to being deflected. A tennis ball delivered to a single point from a single angle with different rates of spin. Replace the amplitude of the wave in the diagram with changing rates and direction of spin.
Wrong. It bends due to Huygens principle, look up Snells law through huygens principle and you will see that it all depends on the angle of incidence and the lights speed in that medium, this video explains the latter
14:10 - if the wavelength shrinks, the frequency increases, so each photon has more energy. This looks like it violates energy conservation, no? Unless the light intensity there is less than before and these two effects balance out? Is this what the explanation is here? 16:28 - also with xrays, this would imply causal action can now travel between events between which it couldnt without the help of xrays+glass. How do we solve this paradox? My explanation was that the very first part of the wave travels as if no interference happens, and the "speed up effect" only happens to the parts of the wave that follow. Because the electron's EM wave isnt actually ahead (in phase) of the incident wave, but is so far behind that it looks like its ahead. This would mean the very tip of the incident wave remains unaffected? No clue if this is right so please correct me The reason i said the electron EM wave cannot actually be ahead of the incident is because electrons take some time to accelerate once the electric potential at their position is changed by the incident wave. 23:04 - if the further-away electrons are not in the right positions wouldn't that mean that the blue wave isnt in the exact correct phase difference to the incident wave, hence there wont be equal amounts of constrictive and destructive interference. This suggests that things like the density of the glass (or any property that affects the arrangement of electrons) messes with energy conservation. Apparently the quantum version of this model is more precise in the details but i havent looked at it and i probably won't understand it. Just one last thing - Ive heard people say the group velocity is lower in glass but the phase velocity is constant. Is this true? If the light isnt actually slowing down (or speeding up), but the pattern of the wave is pushed back, then information should still flow slower. Imagine an experiment with a detector behind a block of glass, measuring the speed of information in glass. With visible light, because the electrons' EM waves lag behind, the tip of the incident wave will make it to the detector as if there wasn't glass, right? But what about after a while - wont information flow slower? For x-rays the tip also takes the expected time as if there wasnt glass. But for information in the rest of the incident wave - wont it make it to the detector faster than if the glass wasnt there? That's what the animation in the video suggests.
V= frequency * wave length To keep the lights velocity same, according to your demonstration, wave length reduces or increases (depending on the place of generation of constructive waves?) So i think when new waves length get shorter thn the frequency/oscillation should get increased and vice versa. But it does not support energy Conservation.
It bends due to Huygens principle, look up Snells law through huygens principle and you will see that it all depends on the angle of incidence and the lights speed in that medium, this video explains the latter
Mahesh: You might be loosing me at this point. Me: Mahesh, my friend, that's simply untrue. Anyway great content, even though I thought I might end up blaming you for taking away time from my gaming session, I just can't at this point. Yeah, I'd be super interested to understand dispersion of light using this concept, would help solidify the concept itself too.
Sir thank you for the clear explanation of the speed of light in medium But i have doubt sir ,how the light bend in travel to rarer medium to denser medium please explain sir
If light is always travelling in a vaccum(cuz atom is mostly empty spaces) why dont light travel through wall? Mahesh pleasee answer ive watched all of ur vids and is a huge fan🙏🙏
Great video as always, Mahesh! I have a doubt-when we talk about lag, does it only refer to the phase? Because if it referred to something else, it would imply that visible light takes longer to travel through glass than through a vacuum, and by that logic, X-rays would take less time in glass than in a vacuum, which doesn't seem correct.
Light goes slower through denser objects because more calculations needs to be done by more instances of charge. Same signal passing through more initial accelerations per unit distance. Therefore the initial to speed stays low. Less dense medium, more room to accelerate faster signal transduction.
@@robertdeland3390 Very simple. Have you ever seen an Olympic event called the relay? If you give more space to each relay runner and they accelerate to a higher top speed. Pack each runner more densely and their top speed goes down because no room to accelerate before they pass the baton. This is exactly what you see.
Your explanation in general is correct and I congratulate you being one of the few giving the correct explanation but IMO not entirely. The laser visible light wavelength is too large to make the electrons inside the atoms (glass is a dielectric it has no free electrons) in the glass to oscillate similar to the photoelectric effect) . It is not a dynamic EM wave interference effect but simply the quantum electrostatic scalar matter field of the glass material compresses the wavelength of the light inside the glass. In the case now of the X-Rays there indeed we have oscillation of the glass electrons and light waves interference effect that changes the phase velocity (not group velocity c) of light inside glass which becomes effectively faster that c value (refraction index n
I think where I'm lost is that this explains how the phase of the light changes, but not really where the front of the wave is. Your animations showed the front of the light getting pulled back or pushed forward along with the phase, but it's not explained why.
It bends due to Huygens principle, look up Snells law through huygens principle and you will see that it all depends on the angle of incidence and the lights speed in that medium, this video explains the latter
Why is it always the channels with fewer subscribers that have the best explanations? You could not have explained this better!
Do they give Nobel prizes for youtube videos? Because you're blowing my mind over and over again...
I think 1 thing is missing here: the actual speed of light in the usual sense is not the phase velocity, but the group velocity at the front of the wave packet (which determines when the first part of the wave packet hits something). The light wave didn't exist for all time, but the 1st part of the light wave reach the electron at some time, and only after that the electron can also oscillate. So even in the case of x-rays, the group velocity would still be the vacuum speed of light, but the refraction is determined by the phase velocity, which is why x-rays are refracted away from the vertical.
ua-cam.com/video/KTzGBJPuJwM/v-deo.html
Thank you! I was just about to make a comment asking what the difference between "apparently" moving faster, and actually moving faster is, since he showed the front of the wave actually being pushed forward past the front of the wave in a vacuum, and that didn't make sense to me. Although he did make sense of some other things, this was the missing puzzle piece.
Has someone actually checked the group velocity of x rays in glass? I'm sure someone has, but I can't find it.
@@alexjohnward The Group velocity of light will slow down in glass and water. X rays and Visible light will slow down the same amount, but their Phase velocity and the angle of refraction will differ based on wavelength.
@@alexjohnward Nevermind, I've been reading more about Optics, and the Group velocity is dependent on wavelength. keep in mind that dispersion is talking about wave packets spreading out, like the envelope is getting broader and may be moving slower than the Phase velocity of the oscillating waves inside.
(Wikipedia, Dispersion) "If a light pulse is propagated through a material with positive group-velocity dispersion, then the shorter-wavelength components travel slower than the longer-wavelength components. The pulse therefore becomes positively chirped, or up-chirped, increasing in frequency with time" So an x ray should move slower than longer Wavelengths.
(RP-photonics, Zero Dispersion Wavelength) "Many materials have only one zero dispersion wavelength within the transparency region, with normal dispersion below that wavelength and anomalous dispersion for longer wavelengths. The group velocity then has its maximum at the zero dispersion wavelength. There, a light pulse travels with highest speed. For example, fused silica has its zero dispersion wavelength at 1.27 μm. For other optical glasses, far shorter values are common (often in the visible range), and more than one zero dispersion wavelength can occur."
Wow!!..blown away ...thankx mahesh...u r the best educator ever i seen on youtube...no one explained that way,they spoon feeded on this platform about light
My mind was blown when I read it as well. Appreciate the comment bro :)
I like how you actually wave your hand when you get into the hand waving explanation.
That's one of the best videos on the internet! I really appreciate your work, because we don't have this kind of depth in high school's physics (speaking from Brazil here), so it's really great to understand it properly!
I am glad you love it. That’s the goal of this channel. To provide deep intuitive explanations! :)
This is the absolute best and most rigorous explanation to be found anywhere on the web, or in any physics classroom (and I have a PhD in physics). Fantastic presentation.
I love how u used topics that are familiar with everyone to describe this crazy phenomenon .Keep up the good work.
After 3 years I finally got my answer, I have been asking this question to my teachers for ages but they just said ' THE LIGHT SLOWS DOWN 🤓' and if I asked why then some of them said ' LIGHT COLLIDES WITH GLASS'S ATOM AND LOSES SPEED 🤓 ' theni get frustrated and asked them WHAT ABOUT EINSTEIN, HE SAID THAT SPEED OF LIGHT (ie C ) IS CONSTANT IN ALL SITUATIONS , then they just ignored me.😅
Thanks Mahesh bhaiya
( Btw I'm still in school 😢 )
"WHAT ABOUT EINSTEIN, HE SAID THAT SPEED OF LIGHT (ie C ) IS CONSTANT IN ALL SITUATIONS" That is not at all what Einstein said!
@ricomajestic * I meant in any frame of reference in vacuum
You sir, are the utmost representative of intuition and simplicity, just like Feynman. I'm pretty sure if you were to be involved in research some 30 years later (teaching higher students), humanity would be thankful to your contributions! :)
I suspect this is as good an understanding as possible without introducing the maths. Very well done.
After watching your video its my time to showoff my knowledge in physics to my friends....i am sure that i blown their minds exactly like what you did to me 🤯🚶🚶💯
Haha, great to hear that! Go Varsha go :D
Me too
Great discussion. When you asked how the light affects the electron, my first thought was it should be shifted by 90 degree. The electrostatic force of the light wave on the electron is maximum when the wave is at a peak/ valley. But that means the electron is 'accelerating' maximum at that moment. With simple harmonic oscillators, the acceleration is max when it pauses and reverses, i.e. when it momentarily stops. And the 'wave' formed when it's not moving is zero and it would be maximum when the electron is moving fastest (just as the original electrostatic force is crossing zero).
That would make the wave formed by the oscillating electron, 90 degrees out of phase with the wave that forces the oscillation. Of course, electron orbitals are probably much more complex than simple harmonic motion, but something at least to think about.
Wow. I found your videos just recently. I have an engineering degree from a long time ago and I wanted to understand special relativity. I have some textbooks. I’ve watched a lot of different videos. Your videos are outstanding. Thank you so much!
I just wanted to take some time to point out how useful your videos have been not just to my to my knowledge of physics but also my perspective on learning. It is easy to believe a subject or topic is boring and not worthy of learning just because of who is presenting the material, without taking into account that everything can be made interesting with passion and love for something. Thank you @floatheadphysics and I hope you continue to make videos
This explanation and insight was great. Thank you.
Glad you liked it
Best and fastest and most intuitive.explanation I've found on internet about the ligth transmission through a material. Before your explanation I had seen unclear explanations from some other well known physics youtubers, and I was a little clueless. Thanks for your clarity and simplicity. I really appreciate it, since it's not easy.....
Found your channel very late...But happy😊😊😊😊😊We want more people like you for reviving Scientific temperament in India
I truly appreciate your content that you put on youtube,,,you are one of the best teacher delivering quality content,,,please do post more videos including dispersion of light ,i would love to have it...
Thank you
Very good. The video from Fermilab got me halfway there but I was still unsatisfied because he didn't go into how the delay of the phase of the forced wave can vary depending on frequency. The x-ray thing too was surprising. I am now satisfied to the extent that I want - I don't feel like slogging through papers because I'm way too lazy for that! Thank you much!
Same here. I thought both the fermi lab and the science asylum videos were incomplete.
What about the Sixty Symbols video?
ua-cam.com/video/CiHN0ZWE5bk/v-deo.html
@@craigtevis1241Ray Fleming has an interesting video worth checking out on this matter where he addresses what is wrong with the fermi lab video.
bro physics with you is art ! No video explained why light is delayed in a convincing way except this one, you made the difference by going into the details with is great, the details really make ALL THE DIFFERENCE, it's like to show someone how a home was built brick by brick instead of showing him how the home was built floor by floor, the brick by brick explanation method is the best and actually you are the only one I found explaining
why the light slows "brick by brick" ( with great details). Keep going like this bro you are doing awesome work, and know
that it's DETAILS that always make the difference in explanation ( explaining step by step without jumping steps). BTW I also think the same way as you with intuition, I found myself asking to me the same question you asked to yourself and you thanked the same way as I tried to do !
@19:20 you might not know it, but this point here is the most critical piece of the puzzle by far. Light is *_not_* a wave, the EM theory is a "hydrodynamics" of photons, applicable when you have a lot of photons. If you try to follow a single incident photon (with a gedanken Heisenberg defeating microscope!) it may not even emerge, it could just get absorbed, or Compton scatter, or small chance for whatever other scattering cross sections. Your wave effect explanation really is beautiful, but some minor inconsistencies some listeners might note could be resolved by knowing that it is only a statistical mechanical explanation. EM fields are fictions. Albeit very useful.
The quantum aspects are totally different to the classical EM wave. These are not at all the same sort of wave. In fact the quantum wave function does not even "live" in physical space. It is not an E and B field.
I love how this topic has been covered by you, 3blue1brown, and Looking Glass Universe, and you each have different ways of approaching how to explain this complex phenomena. Thanks for doing this work and bringing such passion to every video!
And they are all wrong!
This is a brilliant explanation of something I had never even considered. It's almost 60 years ago since I was in college studying Microwave theory, which covered a lot of the same ground - wave phases adding or subtracting - but it was on a space commuications course (NASA), and we didn't go into optics. I knew from general interest how light refracts in glass, and I had always accepted on faith that it is because it slows down in media denser than free space, but the idea that it doesn't slow down - it just takes longer to traverse a glass block of fixed dimensions - is a relativistic thing - and Relativity is not intuitive. But I didn't know that X-rays refract away from the normal.
I can't say that it is all plain and clear to me now, but I can, at least, follow the argument. So refraction in glass, then, is dependent on the wavelength of the wave being refracted. I can't see if it would be directly proportional (I suspect not), but I'm sure there is a fairly simple mathematical expression that relates wavelength to angle of refraction.
I've just subscribed and become the 565th person to give it the old thumbs-up, and I'll be looking for your other videos. Clear explanations of scientific concepts are few and far apart! I've only seen this one video of yours, but I put you in the same bracket as Feinman - and, incidentally, Eintein, whose book "Relativity" (ISBN 0-517-02530-2) is also a brilliantly simple, non-mathematical explanation of a subject so complex that when he first proposed his ideas on Special Relativity, only a few other scientists of his day could come to grips with it.
I'd like to ask some questions, if anybody can answer them ...
1) Are there transparent materials other than glass where the natural frequency of their molecules can be determined (eg water, or perhaps dense gasses), that obey the same refraction formula, based on frequency of the incoming wave, and their own natural molecular frequency?
2) Has the time for light to traverse a block of glass been experimentally determined? Does it back up this lecture mathematically?
Hello sir, I got truly fascinated after reading your comment. Like how the majority of comments here are of Indian high schoolers [ 10th grade, which I am in also now] so this is my attempt to solve the questions you asked.
1. The thing is that glass since glass in it's latent form has excess electrons [ which can be recalled from the rubbing of silk and glass rod ] so in the video, he does not clearly show the happenings for the formation of another wave outside of the light wave because I think that it is very complex to understand that part so for the general audience it has been kept as unnecessary headache to teach.
But the thing about glass is that it is uniform, so the late phases happen uniformly which leads to rarer to denser refraction. But in the case of say water or dense gas, then yes the laws of refraction says that the incident ray and the refracted ray all lie in the same plane, so the refraction formula is already there in the form of snells law, or refractive index according to the speed of light in vacuum.
Those things are in my own school textbook so it will be easier to explain the formula in regards to snells law since our curriculum does check the basics and easy to understand concepts only for 10th grade level since we have social science, biology, mathematics(mainly geometry, introduction to trigonometry and statistics) which just covers almost everything basic of our high school. Then in grade 11th and 12th[ Indian system of classes] we have choices of medical or engineering in the science field, where in the pre engineering classes we will be taught calculus, advanced trigonometry and other things In mathematics, while in physics and chemistry it is thermodynamics, kinematics, optics, electricity, atomic physics and much more, and in chemistry we are taught about the atomic structure, many laws, physical chemistry, and organic and inorganic chemistry to prepare us for the college courses.
So to sum it all up, I still couldn't understand the topics very well that have been taught in this video, cuz physics and mathematics requires a hierarchical system of understanding, cuz without the basics, you can't understand the system presented by mathematics and physics. So I would like to learn all of this in detail later on in my higher studies but that is all I can answer for now
@@VictorKashyap-ie3zq Thanks very much for your considered answer. Most answers I get are two lines long, or simply "lol" or some other thought-provoking wisdom of the sort.
There is another video that shows the wave interference in a better way, I thought, but his conjecture is that light is, indeed, slowed down in glass. Still, if you have time, I think the wave illustrations are good, and show much the same information as in this video. ua-cam.com/video/CUjt36SD3h8/v-deo.html
I saw another video that claimed that light could be slowed down to 38MPH! Look up "How to slow light" I can't remember where I found it, but I don't think it was on UA-cam.
Something that occurred to me about *_this_* video is that at 2:52, he talks about light going through a thin piece of glass, and he talks about the phase change of the wave that causes the light to refract, and the explanation "hangs together" - except that it moves from that virtual thin piece of glass into the next virtual thin piece of glass, but if it bends as he says, you would expect it to bend in each virtual thin piece of glass, so that it would go through the glass body in a curve, eventually getting lost in a spiral. Well, we know that doesn't happen, and I'm sure the lecturer (FloatHeadPhysics) does too, but whatever happens only happens in the first virtual thin piece of glass, where the light enters from a different medium (air, usually). It is dependent on the _interface_ between air and glass - not on what happens in the glass itself. However, I do agree that if X-rays are refracted *_away from_* the normal, then some other explanation than the speed of light has to be sought. I have asked a question on the other video about that.
To be honest with you, though, I don't think I'm equipped to argue this case. I learned microwave theory and semiconductor theory (atomic structure of insulators and doping to produce semiconductors) a very long time back (I'll be 79 soon), and I've partly forgotten, and atomic theory has moved on a bit also. Now, for example, they are starting to doubt the structure of atoms - they aren't a nucleus with electrons flying around them any more - everything is *_fields._* I dunno - but then - I don't have to. In 5 years, I'll probably be gone - and I'm OK with that.
I wish you well for your continued studies. I'm sure you'll learn much more than I ever did. As in the song by Louis Armstrong ...
"I hear babies cry
I watch them grow
They'll learn much more
than I'll ever know
And I think to myself
What a wonderful world!"
Just a thought - could this phenomenon (refraction, and the bending ot the beam) be illustrated in any way with a compression wave like sound? It would be a lot easier to study, and to visualise.
@@DownhillAllTheWay hello sir, tommorow I have my mathematics exam, so tommorow when I will get time then I will do my studies on it, because my syllabus has already been completed, so getting the chance to answer and research these has been feeling great and improving the knowledge.
The song you had written has given me an awkward feeling, not of remorse or sad or happy or grateful. Just an unexplainable feeling of talking to a person who is not scared of his death, or that me being a young guy has not experienced life so for me I will welcome death but after when I fulfill my desired time of life, then I will not regret much, just like you
Seriously impressed with this explanation - thank you, Mahesh.
There's a little problem in 16:40 time mark. It shows like the X-ray passing through the glass can reach the right side of the screen sooner than the X-ray passing through air.
A more reasonable explanation is that the X-ray phase is delayed by more than a half wave per wavelength of propagation, creating an illusion that it was sped up less than a half wave. Hence, actually the X-ray propagates through glass even slower than ultraviolet light, which is slower than visible light.
Let's say that the wavelength of the X-ray is 10 Angstrom, and the atomic spacing of the glass is 1 Angstrom. The phase of the X-ray is delayed by 7 Angstrom after penetrating the glass as far as 10 Angstrom. It creates an illusion that the phase is sped up by 30%. In average, each atomic layer of the glass delays the phase by 0.7 Angstrom.
I completely agree. Look how he gets there.
(around @15:00) He says "there is another way energy can be conserved, that is if the blue wave is ahead of the yellow one"
This is of course not physically possible. The effect cannot precede the cause.
I also don't think he uses energy conservation as an argument correctly.
He first says the amplitude of the yellow wave remains constant because he assumes the glass has no absorption.
And then uses energy conservation to argue the blue wave cannot be pi or 2pi phase delayed.
Where does the blue wave gets its energy from?
Superb way of explanation... wonderful
Boom! I know this feeling my brother!
It is the joy when you understand something so complicated. It is the beauty of Science!
I have struggled with these concepts for years . Thanks to your explanations, physics has now become so much more interesting, and easy to understand.
I have watched so many videos trying to understand quantum physics but believe me you are the best .
Keep up the good work.
Super.... superbbbb brother. I gotta found your Chanel and watching almost all the videos up here.
Your understanding of physics is in another level
From my school life i was curious and also tried to feel , understand what is happening and how.. Bt due to proper resources i can't think like how i want.. Then i shift from physic department to engineering bt i cant forget physics from my heart
However u r a genius person like me😁😁
I would say that the more intuitive way of thinking of the xray situation isn't that it pulls the wave forward, but rather it seems to because it delays it beyond a half wavelength so it appears to move the peak forward through a kind of wagon wheel effect
To me this sounds more logical, otherwise we would be able to deliver information faster than light.😮
i used to think that photons had to move through a more zigzag way due to more congestion of particles which is greater in length compared to a linear distance,and then we were to consider its velocity through a denser medium as the ratio of its linear displacement to elapsed time which will obviously be smaller.Watching this video helped me think how wave nature of light can be fitted the same way,thanks!
God has graced you with a very intutive mind keep it up
actually you are my best UA-camr
I can't believe how good this explanation is. Amazing
I always had this doubt in my mind that why is it said that light slows down or sppeds up because according to the Special Theory of Relativity, the speed of light is a constant, irrespective of the medium it travels through and then I would think what happens during refraction. I even asked my teachers but didn't got an answer. But now I know the answer just because of you! I really thank you soo much for this video and all the other amazing video that you have uploaded (also on Khan Academy) I just had one question, why does the light bend? The light wave just got shifted backwards due to the resultant new wave but why does it bend away or towards the normal?
"the Special Theory of Relativity, the speed of light is a constant, irrespective of the medium it travels through" Einstein never said that and it is not in the Special Theory of Relativity.
Very nice and complete explanation
Not sure that the X-ray wave is pushed forward, that would be the wave influencing the electron before it gets there. Tather, the X-ray wave is so small that the influencing wave is pulled back beyond the crest of the wave, resulting in the phase of the previous wave appearing in the next wave. (if that made sense)
@jimjohnson394 It is not that the Xray wave gets pushed forward. It gets pulled back so much that it is forward of the NEXT cycle.
Great And Great Explanation!! Great Work!! the besttt explanation! Specially without maths, Bestt!
I still don't understand how this resulting phase change of the light causes it to bend 🤔
Also, shouldn't the electric field of the electron also affect light behind it once it starts oscillating?
Is it possibly since light has only it's own phrase of rephrase through space. Always moving forward
Agreed! And the "energy conservation dictates phase change" description ignores the energy transferred to the electrons to get them oscillating kinetically in the first place. And if materials can be considered as being essentially a vacuum in this (glass) case, why not in all cases - why don't all non-metallic amorphous materials transmit visible light then? Mahesh is brilliant but this video raised more questions than it answered.
I'm with you. My *guess*: remember the light bend towards normal only happens when the light wave is at an angle to the glass -- if the light enters the glass perfectly perpendicular to the glass, it propagates through the glass with no angle change. The higher the angle of the light entering, the more the turn towards normal -- and I suspect the difference in the number of atoms on each side creates the imbalance that results in the angle change. If you consider the lightwave just a fraction of a moment after it enters the glass at an angle, there's just one glass atom above it that starts oscillating, whereas it's already got many atoms below it that are oscillating, which if you did the vector math, the imbalance between the induced waves of the one atom above and the zillions of atoms below, causes both the phase change and angle change. That's why light entering at a more oblique angle sees a greater bend towards normal, less oblique sees less change, and with a perfectly perpendicular beam feels perfectly symmetric induced waves on either side (so it gets a phase change but no angle change)
@joet9267 i do think one of the things that was missing from his explanation, in order to simplify it, was that he is only showing one propagating wave but in reality there would be lots and what we see is the probabilities of all them and all their interactions combined which is what causes the change in angle. I say this because otherwise you would not see the same phase shift when light entered perpendicular to the surface of the glass. so the phase shift and the angle traveled would have to have different causes if it happend at all angles. But as i write this i realize that still would not explain why x-rays bend the opposite as light waves based on the imbalance of oscillation idea joet9277's idea and i'm still not convinced i understand this whole phonominum. What am i still missing or misunderstanding? anyone have further thoughts?
also, even non transparent materials are mainly empty space so why do they reflex light instead of letting light pass through mostly unimpeded. I would suspect it's more about the field interactions, which are way bigger than the matter itself, and orientations of the molecules themselves. I would love further help to understand this whole topic. i have clearly missed something or there is still missing information.
I love your video! Thank you for this work!
However, it opens new questions:
1) If the wavelength in glass becomes shorter then it should become shorter and shorter for thicker glass. And as far as I know, this is not what happens in reality. so, what is going on?
2) I was taught that photons deliver their energy to electrons only in packest of energies, shifting the electrons into different energy levels around the nuclide. So, how can the photos deliver just a bit amount of energy to the electrons?
Thanks again
2) You're mixing up two things. The quantization ("portions") of electron energy is a concept for the "orbit" the electron is in.
We don't change the orbit here, we just slightly "wiggle" the electron on it's orbit by it "feeling" the electromagnetic field that is the photon.
it need to get pulled back in every infinitisimal glas-strip, to continue the slower path. if it would stop to get pulled back it would go at c again
I am amazed that it still travels at C. You didn’t emphasize that this also caused the change in direction. I am a bit on clear how the pulling or pushing changes the direction. I really understand better with your explanations.
To have spectrum from prism, light is bent due to inelastic collision, massless of light becomes mass particle as you can see the light on the surface. Re-absorption of energy happens at the same moment for light to bend in order to achieve light speed, visible least energy of absorption is red light energy due to least bending, gradually visible higher energy of light absorbed due to bigger bending, such as violet, that is how the rainbow is formed. There is no light interference in double slit experiment, the formation of pattern is due to multiple source of light rays after through the diffraction grating plate, you can try by making 2 rays from same torch light and shine towards double slit plate, and cover one of the rays for your verification . There is light diffusion and there is no light diffraction. By shining light to a bead., you can achieve centered spot of image for small round bead, but you will get off centered image spot for larger bead. microwave and X ray are transmitted by alternating the current, the alternating current of x ray is faster than microwave. Imagine a small ball, you hit with 2 hands, continuously by applying same force, you hit the ball with open arm by imagine it is microwave, and you hit the ball with short distance gap by imagine it is X-ray. The energy apply to X ray is higher in this case. that is the reason why microwave can heat up matter due to hitting atoms up and down with each other, whereas X-ray is able to penetrate matter deeply, the atoms in the lattice experience very quick vibration without atom hitting each other and remain in the lattice as they are, then clear X-ray photographs is achievable. Light does not travel through wavelength or frequency due to massless. light travels at light speed regardless of medium or type of lights.
Interesting point of view. I'll need to think about it more, I'm not on board w/it, especially the xray theory but understand the thought. In my mind with xrays being much higher freq, the electron will only be 180 out of phase every so often, not for each wave. The video post; The Primer Fields Part 3 may interest you, maybe check out. The xray reaction has issues in the Primer Fields too. Excellent subject, look forward to having some extra time to think about it. Nice post. Look forward to reading the comments.
I really liked your explanation.
Mahesh... loving your channel. Would love to see you give your intuitive explanation approach to the similar phenomena of RF in various antennas. Thinking Yagi Udas, Cassegrain reflectors, phased arrays, long wires, and small transmitting loops. A discussion of directivity and impedance matching would be amazing.
Excellent work! Thank you!🌈
Back when I looked at the phenomenon of light of different colors being diffracted as it entered mediums that were optically more or less dense , it always annoyed me that the textbook answer , more or less , was that the optical light is " slowed down " as it enters , say from air into glass . I was annoyed because I knew that light cannot " speed up ", or " slow down " in a medium - it must always have the speed c ( c= speed of light ) relative to the observer . Your explanation goes a long way toward clearing that up for me . And very interesting as well .
So, what I'm understanding is that refraction is redshifting/blueshifting the light; by changing the wave's length, it's changing the ...'type' of light. So in the example of X-rays, it would be shifting the light inside more towards gamma rays, yes? Redshifting/blueshifting of light occurs for the preservation of c... but if refraction needs the oscillation of electrons for refraction of a light wave... what causes a counter-wave/wavelength change necessary for redshifting/blueshifting within a vaccuum as the distance between distant celestial bodies increase/decrease?
Amazing explanation ! 👌👌🌹
Than you for this cool video. It blew my mind also as you showed that with the conservation of energy you can guess the position of the secondary wave. AMAZING :)
But here 2 additional informations:
1. As the electron moves up and down it emitts the Photon with a phase of 90° later according to the displacement. So the blue wave (secondary wave) is in the minimum 90° behind the yellow wave (primary wave) up to 270° behind the yellow wave.
2. Optics books never explain the situation, what is happening during the time as the first wave hits the glas. It always describes a steady-state situation: The light wave already passed through the glas, as we see it here in you explanation. So the claim of beeing faster then light ist mathematicaly correct, but the information actually already passed through the glas cause the whole glas is already filled with a light wave. So no information is moving faster then light. Never saw a mathematical explanation what is actually happening as the first wave hits the glas.
consider the analogy of exciting one tuning fork with another tuning fork of a different natural frequency. Energy conservation constraints will require the excited tuning fork to ring at the same frequency of the exciting fork, but somewhat "quadrature" in phase. Amplitude of the ringing of the excited tuning fork will depend on the difference between the natural frequencies of the two forks.
Wonderful explanation sir ❤❤🔥🔥
I loved it.
Thanks, Omal!
Superb explanation ever
Hey, I have a question.
If the waves are Electric Fields .. then how come does they feel a pull or push , which a inertial
property.
Would wish you would use a pop-filter in front of your mic. The popping sound can become distracting especially listening with headphones. Very interesting videos. Love them.
Perhaps this could be extended to explain why glass is good at reducing UV rays? Well done
6:25 why isn't the blue and yellow constructively interfering?
did you get it? after 9 months of commenting, or u still dunno?
9:15 there you go lol
Thank You sir I was really in doubt about this matter that does light acctually slows down ur vedio has helped me a lot to understand it. 😊😊😊😊😊😊
Excellent good job !
I like the fact that you reply to all the comments. Btw your class 10th science video is helping me so much.😁
Awesome to hear that, Shail! :)
@@Mahesh_Shenoy oops! It's ‘Sahil’ not ‘Shail’ XD. I remember my friends used to write my name as ‘Shail’ instead of ‘Sahil’.
@@GodSahil that was autocorrect!!
@@Mahesh_Shenoy oh! XD. Np
21:07 can we just say that the electron in the mean position is in Direct influence of that electric field and the electrons above and below will get to the direct influence of electric field but after some tiny time delay.... 🤔
It is visible in the animation.... May be (mostly) i am wrong but it gives me a little more satisfaction 😅
Not really! Mainly because, and I forgot to mention that, we consider the EM waves from the source to be plane waves!
@@Mahesh_Shenoy i understand....
That's the main reason i said i am mostly wrong...
But we can perhaps say that the intensity of the electric field is more at the mean position than other positions which causes different acceleration to the electrons in different positions which may results in the case above in the video.... Probably 😁🙄
I literally studied only high school physics and currently in that phase preparing for JEE Which is in next 23 days..... And yes your channel helped me amazingly a lot in visualising the physics.
@@Mahesh_Shenoy or am i still wrong sir?
@@nothingspecial9370 plane waves have same intensity :D
My own, admittedly, imperfect analogy is to think of crossing a chasm on a bridge made of swinging boards. The boards' swing frequency can not be changed and the runner's step frequency can't either. However, long legged runners swing their legs slowly and short legged runners swing their legs fast. So all runners have the same speed (lambda times f = c). A runner who swings his legs slowly may find himself hitting the boards when they are going back and so is always delayed. A fast swinging runner may hit the boards when they are swinging forward and, thus, crosses the chasm faster. It all depends on matching the leg swing frequency to the board swing frequency.
If a runner's leg swing just matches the board swings he may end up not feeling the boards at all and his timeing to cross the bridge would be the same as with non-swinging boards (refractivity index of that bridge would be 1 for his frequency). If the board swings are just as fast as the leg swings, the runner may end up going nowhere and it could be said that he has been absorbed; as in red light not passing through blue glass.
There's a reason why swings are always used to explain resonance frequency.
Great video 👍
Ohh Wow What a great explanation ever Thank U thank u thank u very much I am very very happy now. I am having a confusion on it ❤❤❤😁😁 keep going.. 🙌👍👍👍
It makes sense how the light gets pulled back but why does it bend? You should something like light is going forward but an effect of coming back shows. Like when a wheel moves fast forward and it looks like its moving back or in a fan sometimes or a broken tv screen. Sorry it might be dumb question but wanted to learn more
It bends due to Huygens principle, look up Snells law through huygens principle and you will see that it all depends on the angle of incidence and the lights speed in that medium, this video explains the latter
Ιt's like pushing a ruler into water at a constant speed but at the same time the ruler lenght inside the water shrinks! Similar to the wavelength of the light that shortens inside water compared to air. Therefore, although the speed of light (photons) does not change and is always c between the vacuum inter-molecular space of any medium because the ruler length shortens as it goes inside the water it will take more time to reach the other end of the water medium and exit. So effectively the speed of light goes slower in water υ=λf with f frequency being fixed and wavelength λ getting shorter in water therefore effectively the speed of light inside water υ is slower.
Fantastic video! Best explanation I've seen!
I didn't understand the part you said the electron and light wave wouldn't be in the same phase because of energy conservation. Do the electrons say I better not be in the same phase because i don't want to break the law of conservation? What's the mechanism there? I am a little confused.
When passing light, does the temperature of the glass remain completely unchanged or does it heat up? If the glass shows an increase in temperature, wouldn't that indicate absorbation and dissaption of energy from the light source?
Question: If visible light bends downward through a medium and X-rays bend upward, is there a frequency that can travel unaffected through the medium?
Hi Mahesh, your channel is my favourite for understanding physics from ground up, intuitively. You are awesome at what you do. I have a small stupid question regarding this explanation and I would be obliged if you do take time out to address my query. You say that an infinitesimal strip of glass does its bit to 'pull a wave back', so the more number of such strips that we consider, more is the 'pull back'. So, the 'apparent velocity'/'perceived velocity' of visible light decreases with the increase in the number of such infinitesimal strips. In other words, the apparent velocity must somehow depend on the width of the glass cuboid. But we know from high school physics that the apparent velocity inside a medium depends only on the medium and not its geometry. Is there a fallacy in my understanding because I am unable to reconcile these two?
love the explanation
You could have put diagonal electron waves in the picture to show how the lagging works or draw some kind of triangle to show the length difference. But that was the only part I had to skip to because I didn't understand it immediately. And the fact that I understood the rest what you said very well shows that nonetheless your explanation was really good. Great video👍
Your explanation is amazing. But I still have one question, it might be silly though. When you demonstrated light passing through the glass layer made of 1 electron, it made sense. But when there are multiple layers of glass, thats when it got confusing for me. Because if we have multiple disjoint layers of glass, the light wave would show multiple refractions. But if we have a big glass cube, it will only have a single refraction. But according to your explanation, a big glass cube should also show multiple refractions since it would have electrons one after another just like disjoint slits of electrons as shown in the video. So why do we see refraction only happening once in a big glass cube instead of multiple small refractions as shown in the video? sorry for my bad english but this question is confusing me a lot. And thanks for the amazing educational content!
Bro I love watching your videos. Please continue making more!
Sir i think this was all about the speed of wave which you explained beautifully and someone has a good knowledge of interference can understand it easily but you did not explained why it bends? Or did I missed something 👀
No, I didn't explain why it bends. Wanted to do a follow up video which I never got to :D
I would be interested to hear if you have any similar explanation of the bending of light seen in gravitational lensing? Also why certain materials are transparent and others are opaque to light and why some reflect it.
I have an observation and a question/request for an answer. I take it at the right UV frequency there is no refraction?
I would like to see how the light bends back at the end of the glass if that is a vacuum? And intuitively it makes sense to believe that the wave would carry on the same as it did before in the glass as there are no electrons for to interfere with in the vacuum but that is not the case as it bends back to the original path?
Intuitively it seems more like the beam is getting nudged to one side but still attempting to travel the same direction hence it comes out displaced but still pointing the same angle as it entered the glass?
thank you so much for great explanation
❤- Thank you Mukesh - Would you consider making a series on gravitational lensing? - Eddington's photographs of the 1919 solar eclipse and what causes the observed deflection of the light source from distant stars by the Sun. What does Fyman say about this matter?
The actual bending, not the slowing, is result of phase and vectors acting on the wave. Though the light can approach at one angle, the "shape" of it at any moment makes it more or less susceptible to being deflected. A tennis ball delivered to a single point from a single angle with different rates of spin. Replace the amplitude of the wave in the diagram with changing rates and direction of spin.
Wrong. It bends due to Huygens principle, look up Snells law through huygens principle and you will see that it all depends on the angle of incidence and the lights speed in that medium, this video explains the latter
14:10 - if the wavelength shrinks, the frequency increases, so each photon has more energy. This looks like it violates energy conservation, no?
Unless the light intensity there is less than before and these two effects balance out? Is this what the explanation is here?
16:28 - also with xrays, this would imply causal action can now travel between events between which it couldnt without the help of xrays+glass. How do we solve this paradox? My explanation was that the very first part of the wave travels as if no interference happens, and the "speed up effect" only happens to the parts of the wave that follow. Because the electron's EM wave isnt actually ahead (in phase) of the incident wave, but is so far behind that it looks like its ahead. This would mean the very tip of the incident wave remains unaffected? No clue if this is right so please correct me
The reason i said the electron EM wave cannot actually be ahead of the incident is because electrons take some time to accelerate once the electric potential at their position is changed by the incident wave.
23:04 - if the further-away electrons are not in the right positions wouldn't that mean that the blue wave isnt in the exact correct phase difference to the incident wave, hence there wont be equal amounts of constrictive and destructive interference. This suggests that things like the density of the glass (or any property that affects the arrangement of electrons) messes with energy conservation.
Apparently the quantum version of this model is more precise in the details but i havent looked at it and i probably won't understand it.
Just one last thing -
Ive heard people say the group velocity is lower in glass but the phase velocity is constant. Is this true?
If the light isnt actually slowing down (or speeding up), but the pattern of the wave is pushed back, then information should still flow slower. Imagine an experiment with a detector behind a block of glass, measuring the speed of information in glass. With visible light, because the electrons' EM waves lag behind, the tip of the incident wave will make it to the detector as if there wasn't glass, right? But what about after a while - wont information flow slower?
For x-rays the tip also takes the expected time as if there wasnt glass. But for information in the rest of the incident wave - wont it make it to the detector faster than if the glass wasnt there? That's what the animation in the video suggests.
He is using the wave picture of light. He's not talking about photons!
Hi. Could you share some of the resources /reading material you used while researching this topic. Thanks. The video is insightful.
Check out Feynman lectures!
V= frequency * wave length
To keep the lights velocity same, according to your demonstration, wave length reduces or increases (depending on the place of generation of constructive waves?)
So i think when new waves length get shorter thn the frequency/oscillation should get increased and vice versa. But it does not support energy Conservation.
Interesting. But how does this explain bending? And why metals reflect light so well?
It bends due to Huygens principle, look up Snells law through huygens principle and you will see that it all depends on the angle of incidence and the lights speed in that medium, this video explains the latter
Mahesh: You might be loosing me at this point.
Me: Mahesh, my friend, that's simply untrue.
Anyway great content, even though I thought I might end up blaming you for taking away time from my gaming session, I just can't at this point. Yeah, I'd be super interested to understand dispersion of light using this concept, would help solidify the concept itself too.
Aww! Glad you liked it!
So the phase shift of the induced wave depends on the light's frequency relative to medium's resonant frequency?
Accurate interpretation, very nice. But how does this approach explain the refraction (bending)?
Sir thank you for the clear explanation of the speed of light in medium
But i have doubt sir ,how the light bend in travel to rarer medium to denser medium please explain sir
If light is always travelling in a vaccum(cuz atom is mostly empty spaces) why dont light travel through wall? Mahesh pleasee answer ive watched all of ur vids and is a huge fan🙏🙏
Great video as always, Mahesh! I have a doubt-when we talk about lag, does it only refer to the phase? Because if it referred to something else, it would imply that visible light takes longer to travel through glass than through a vacuum, and by that logic, X-rays would take less time in glass than in a vacuum, which doesn't seem correct.
Light goes slower through denser objects because more calculations needs to be done by more instances of charge. Same signal passing through more initial accelerations per unit distance. Therefore the initial to speed stays low. Less dense medium, more room to accelerate faster signal transduction.
??????
@@robertdeland3390 Very simple. Have you ever seen an Olympic event called the relay? If you give more space to each relay runner and they accelerate to a higher top speed. Pack each runner more densely and their top speed goes down because no room to accelerate before they pass the baton. This is exactly what you see.
Sir , shouldn't the oscillating electrons produce electromagnetic waves propagating in both directions?
Great question! And that's reflection :)
@@Mahesh_Shenoy thanks😇
Your explanation in general is correct and I congratulate you being one of the few giving the correct explanation but IMO not entirely. The laser visible light wavelength is too large to make the electrons inside the atoms (glass is a dielectric it has no free electrons) in the glass to oscillate similar to the photoelectric effect) . It is not a dynamic EM wave interference effect but simply the quantum electrostatic scalar matter field of the glass material compresses the wavelength of the light inside the glass. In the case now of the X-Rays there indeed we have oscillation of the glass electrons and light waves interference effect that changes the phase velocity (not group velocity c) of light inside glass which becomes effectively faster that c value (refraction index n
I'm still catching up and this is perfect
I think where I'm lost is that this explains how the phase of the light changes, but not really where the front of the wave is. Your animations showed the front of the light getting pulled back or pushed forward along with the phase, but it's not explained why.
Then, how do you explain regracive index as the ratio of the velocities
what about gamma ray and other EMW with different frequencies
How the direction of light changes , is it due to phase change in medium
It's got to do with the wavelength shrinking!
@@Mahesh_Shenoy By shrinking you must mean changing. The direction of light changes due to any delta lambda, not just a shrinking one.
It bends due to Huygens principle, look up Snells law through huygens principle and you will see that it all depends on the angle of incidence and the lights speed in that medium, this video explains the latter
From where does electron get energy to oscillate? If electron is getting energy from em wave ,doesn't it effect original wave?