Formation of rainbows (& how far are they) | Physics | Khan Academy
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- Опубліковано 26 лют 2020
- Rainbows are formed by the dispersion of light & reflection (not total internal reflection) from drops of water. The rainbow color sequence can be analyzed by drawing a ray diagram of the refraction of sunlight inside water drops. Get more practice on the rainbow formation here
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Created by Mahesh Shenoy
You do get kudos for explaining that (although you didn't explain why) total internal reflection does not occur. But (1) dispersion is not synonymous with separation of colors, and (2) separation does not explain why we see bright, colored bands. If it did, we would see a rainbow in the light that exits without reflecting (this light is visible, and is called the zero-order glow).
Part (2) first. If you would draw a ray of sunlight that hits the drop a little higher or lower than the one you used, its reflected blue ray can exit the drop at the same angle your red ray did. And the same goes for every color in between. Over most of the reflection, an observer sees all colors combined, making white. Just like in the zero-order glow. This is why the sky inside the rainbow is brighter than outside.
An incoming sun ray that hits the drop in the center is reflected straight back toward the sun. As it moves up on the drop, the angle between the incoming and reflected ray increases, but only up to about 40 degrees. There, it stops increasing and begins to decrease. This change causes many rays to "bunch up" right around 40 degrees, making the reflection brighter there. This effect, called an optical caustic, is quite often seen at the bottom of a mostly-still swimming pool or clear pond on a sunny day. Bright bands of sunlight are seen to almost be dancing around the bottom of the pool, mimicing the waves on the surface.
The caustic in a rainbow is seen at about 40 degrees. But "dispersion" means that all of the colors, which were treated the same way before hitting the drop, must be treated differently after. If there were only one ray (or many that all retract the same way), then separation of colors occurs. But when they refract differently, the effect is know as color aberration. It is commonly seen, when lenses are used to project images on a screen, as a blurring of colors around the edges of objects.
So rainbows are caused by the combination of optical caustic and color aberration. Red appears on the outside because its caustic is at 42 degrees, not because it refracts less
My main problem was the reflection of colours inside rain drops. Now it is clear. Thank you sir.
The reflection inside drops happened instead of refraction because the incident angle made by ray was more than critical angle .
Thank you so much sir, it was really too simple and I understood the concept of rainbow. I often think that how are the rainbows formed and try to search for it online, but get confused because none explains it too deeply as you and no other expert tells any logic for the concept 👍👍👍
Now I can easily explain this concept to anyone
What a great explanation, thankyou!!
i find this video better than all the video i ave so far seen!! U really helped sir...thanku for explaining in so detail..
Amazing; thank you so much for this explanation.
It's the best lecture on rainbow I hv seen till now on utube..u don't need to switch to another video bcz believe me, it's best explanation till now🌟
Thank you so much sir! It was too simple :)
@@cyrusvicente3896 give me
best ever video, i've ever found on any topic..just lit!!!
Thank you so much
GREAT EXPLAINATION!!!
Brilliant content! Really....!
Thank you so much Sir. My concept is much clearer now
Well said. Nice, clear presentation. Thanks.
I get the reflection and refraction in the raindrop part from other online materials. You are best in explaining why a rainbow goes from red on top to violet at bottom, and why it appears circle or arch shaped. Thank you so very much.
It was super duper ammazzzzingggg..... I can't explain what i was finding that "is there not mess of rainbows" but sir you explained it so perfect unbelievably nicely that I just can't explain.....no words fit into your effort...Hats off 🤠🤠❤️❤️👍👍👍🙏🙏🙏
Thank you for nice explanation.
Vry nice explanation sir
Thanks sir I love your voice nice explanation
Congrats on 100K subs!
thank u soo much tommorow is presentation of rainbow formation and my tuation teacher is absent i was really tensed about it
Wonderful explanation 👏👏
Super clear explanation 💝
Sir, you told us that the a part of light refracts before reflecting internally after dispersing in the raindrop. So even if we face the sun why can't we see the rainbow? The colours which refracted should be reaching our eyes isn't it?
Maybe they are refracting but the majority of part is reflecting internally so intensity of light refracting is not that much
And this is the reason why the explanation, based on separation of colors, is wrong. If it causes the bright bands in the rainbow, it would also cause bright bands in the "zero-order glow" seen around the sun. I'm going to write a new comment that explains the correct reason.
God bless you. Rainbows have puzzled me so much. Learnt a lot. Thank you.👍👍👍👍🧠
Fantastic sir.
what a nice explanation
Very good explanation sir
Great explanation sir
Outstanding
Sir I have a doubt ,as you said that so many rain drops in a line makes a specific colour visible d but do'nt the rain drops reflect the light which is coming from forward rain drops
Yes to me also rakesh sir has recommended the video
Bhai kon hai tu
@@vishalbutola5518 arey bhai mai hu anmol
@@shivbholeshivbhole1318 okkk bro
Pollii man
Poweresh 🔥 🔥 🔥 🔥
@Abzar 😂😂😂
I am a teacher and it helped me a lot
Nice
nice picture
hi is christopher
Can u please explain why rainbows are curved...?
0:53 okays🌟
2:00 😄😃
3:20 🙂
4:50 💯
7:30
Sir, but according to the diagram at 12:36, the order of colours in the rainbow is ROYGBIV not VIBGYOR... then why do we see the order of colours as VIBGYOR?
In primary rainbow, in VIBGYOR colour sequence, V is at the bottom and R is at the top....
In secondary rainbow, in VIBGYOR colour sequence, V is at the top and R is at the bottom ...
☺️
At 8:42 sir madr mistake.
We stay at a fixed point. Dispersed lights from only certain drops come into our eyes. Higher raindrops’ red light get into our eyes but not other colors; Violet light from lower drops reach our eyes but not other colors; So do other colors in between. That why rainbow comes in the order of R..V in order
you are great explainer but your voice is low
Why it is forms in a corved shape
Sir please make video on domestic circuit. Exam is very near. Only you can help sir.
How much you got?
At 7 : 34 , the reason that why light reflects in even transparent media is that the angle of incidence made by incident monochromatic light inside raindrop is greater than critical angle and light reflects .
Critical angle is the angle of incidence at which the angle of refraction becomes 90 ° when light passes through the denser medium ( water drop ) to rarer medium ( air ) . Hence , when the incident angle exceeds critical angle , the light refracts but in the direction outside the interface or say axis which results in total internal reflection .
Nope. Look at, for example, the red triangle inside the drop. The two sides made by yhe dotted lines are both radii of the sphere. So the triangle is isosceles, and the angles the ray of light makes are equal to each other. Since the incoming angle is the angle of refraction, it is, by definition, less than the critical angle.
TIR is impossible.
Everything's fine, but how will the raindrops last after rain cuz we only see rainbows after rain.
If the raindrop on top reflect the blue light or any other light then the sequence of colors i.e. ROYGBIV may shuffle and we can't see the same sequence??????????🤔🤔🤔🤔 please explain this in future videos....plzz
Regardless of where you stand, you will always find the raindrops reflecting red to your eyes are higher than the raindrops reflecting violet to your eyes. So if the raindrop that looks red to me looks blue to somebody else (if they are standing at a higher level as mentioned in the video), then for them red will be given by even higher raindrops.
In short, everybody sees their own rainbows :). Hopefully that made sense.
@@KhanAcademyIndiaEnglish
Mahesh sir, ok I understood that every body see there own rainbow.. But still I have couple of doubts:
1.why should always the raindrops which are higher should reflect red light..
2. It is said all the raindrops acts small prisms and they disperse the sunlight, so why don't all dispersed light fall on each other and mix to to give again white light before or after entering our eyes. (as you said earlier in prism video) finally there would be no rainbow...
I think I am the first to get the doubt replied by you. Thanks sir, keep going... You and ram sir are legends of khan academy.
I hope you may also explain this dout. 🙏🙏please...
You both almost created all the videos for tenth. But now we are going to 11th , what about them sir. I got some interest in science only because of your teaching style.
I want to meet you both one day sir in summer for more classes or I will help you to convert the videos into telugu. please send me your address to preetamsonu12345@gmail.com and I promise that I don't misuse it..
Kindly waiting for your reply....
Why there are like a arc
What about the other person to whom blue light was reaching(10:00, in video) ....... Would he see blue light at top? I think no...... But why?
No! Because he will also be able to see red color which will come over blue!!
13:27 2 rainbow are formed
see upward
8:42 mistake
Blue light will reflect below the red.
Rain drops are spherical surface bro
After a long time...............
It is impossible how can change the yellow light in the colours I want the next chapter for this it is very hard to study also it is right
v čierne dieri sú tváre
why let
Wow. I love this
Thank you ....
7:47 how can you say that ???? Even in DOE worksheets it written. how can they be wrong ??
Sir, please make video on Domestic Electric Circuit of class 10th before 3 March...
👏👏👏
Who is here after rakesh sir recommended us😅
Bro mei hu 😂
SIR DOUBT>>>> U SAID THAT RAINBOW IS EVERYWHERE THEN WHY CANT WE SEE A RAINBOW TOWARDS THE SUNLIGHT..... I KNOW THAT BECUZ OF TOTAL INTERNAL REFLECTION ..... WE HAVE TO BE OPPOSITE THE SUNLIGHT ..... SO WHY DONT THE REFLECTED LIGHT GETS REFLECTS AGAIN BY THOSE RAINDROP AT MY BACK
The short answer is you can. There are secondary, tertiary and even quarternary rainbows. The higher orders end up at higher angles, meaning the quarternary rainbow indeed ends up in the same direction as the sun. However, with every reflection, the intensity drops dramatically. So quarternary rainbows would definitely be extremely dim. Also coupled with the fact that you are looking in the direction of the sun means it's almost impossible to see. But it turns out there are a couple of photos of quarternary rainbows. But you can hardly make out anything in them :).
@@KhanAcademyIndiaEnglish okay sir got it! But I have a question that will a person while moving will see the same rainbow as the person standing still..... Or something else for both the person...THANKS A LOT SIR....😀
SIR U TEACH LIKE PROF.
WALTER LEWIN
ｃａｒｔｏｏｎ ｊａｉｓａ ｋｙｕ ｓａｍｇｈａ ｒａｈａ ｈａ.
ｂｈａｉ
Sir Red colour should be on top according to VIBGYOR. But in your explanation it's at the bottom. That's totally incorrect. Please consider. You diagram is wrong. I have been learning through your videos from a long time, so these mistakes disappoints me and others greatly!
Regards
Ishitta Mazumdar
(Your Student)
Red is at the bottom and reaches at a lower place. So the raindrop that sends the Red to your eyes has to locate higher than the one that sends you the violet.
@@hongchai8981 Ohh. But it still means that RED is at the top right? But sir is saying in the video that Red is placed at bottom when rainbow is formed!!! That's what I'm asking.
Bhai galat padha raha hai red color is first colour present in rainbows 😂😂🤣🤣🤣😱😱
That's the sequence of colors that u've understood wrong
VIBGYOR represents from inside to outside
Thank you so much sir, it was really too simple and I understood the concept of rainbow. I often think that how are the rainbows formed and try to search for it online, but get confused because none explains it too deeply as you and no other expert tells any logic for the concept 👍👍👍