Thin Film Interference part 1 | Light waves | Physics | Khan Academy
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- Опубликовано: 7 фев 2025
- Learn how interference of reflected waves causes oil on water to appear all rainbow-y. Created by David SantoPietro.
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I got mad and almost threw my laptop out of the window when trying to visualize this phenomenon in an optics book. You saved me hard. Thanks a lot!
I thought i was the only one
Why do I feel you
Ahhh my man!! Same!!
I just wasted two hours trying to learn this and you explained it in 12 minutes. Thank you.
i love this side of youtube!! im in college for a music degree. i like science, but i dont take science classes because they arent required for my degree. watching science videos online gives me the chance to learn this stuff :D i LOVE it omg
my textbook did NOT clearly explain why the path-length difference was 2t. Your explanation is so simple and clear. Thank you!!
You are probably saving my physics exam. A big thank you for all the videos
Hi from 5 years, 😄
What are you doing now in life?
Hi from 7 yrs....what are u doing now😊
David has a talent for making complicated things easy to understand! Thanks!
Elon 😂
I have a homework question that is asking me to show that the reflection cancel each other out when ns>n1>n0, (vs
I was preparing for physics exam, couldnt understand this topic even in my own language. This video really helped, thank you a lot.
Btw, your voice is amazing.
Good luck!
David is my fav on the whole Khan academy
I was under the impression that you only get a phase shift if you are going from a lower index of refraction to higher. Doesn't that mean that we don't get a phase shift from oil to water because oil has a higher index of refraction?
I think you're right, but there is a phase shift from air to oil, and there isn't another one to balance it out so the condition is reversed.
@Kaleb unknown maybe a bit too late
THANK YOU SO MUCH FOR THIS VIDEO I was struggling so hard, my professor could never have explained it better!!
Same situation for me tooo 😭
blew my mind within the first minute. Always wondered what those colors were in bubbles.
Saved my day❤
Dat cliff hanger do.
But water has an index of refraction of 1.333 (4/3) and oil somewhere around 1.5. That means that since v=c/n, the velocity of the light wave in oil will be less than in water since velocity and index of refraction are inversely proportional if the constant c is constant (which it is, speed of light in vacuum). If water has a smaller index of refraction then a wave has a larger velocity in water.
well dats the point i waz thinkin' about
Katerina Santicola exaxtly 🙄
Please set this playlist .... everything is here and there
The trick is if the wave bounces off a higher index of refraction, ie slower speed, it'll phase shift by pi. If you compare the two bounces, odd symmetry is odd integers of lambda and even symmetry is even integers of lambda.
Holy moly thanks a lot!!!!!!
some of my friends were having issues understanding this videos but just incase anyone else is having any confusion - path length = wave length
Sorry, late reply, but path length and wave length are NOT the same things. You can have many wavelengths (λ) in a path length, as the path length is the length from where the wave starts to where it hits the screen.
Just wanted to put that out there for other people who see the parent comment.
Beautiful explanation.nobady can explain this concept in such a way, except you.
Khan academy is best
THANK YOU!!!!!!!!!!!!
You got it wrong here.
Extract from the article from wikipedia:
There will be a phase shift upon reflection from the upper boundary because n_airn_ water.
Water has smaller refractive index so it is faster medium than oil. (speed = c (speed of light) / n (refractive index)).
+Mark Jarc its the sane thing
elementery class rule when a light is in a denser medium the speed of light is slow as compared to that in vacccum
He said "for the sake of argument"...
But what if the light come in a certain angle? How do you explain that?
why there's a pi shift when the reflection takes place with a slower medium??
I don't if you are still wondering about it but the answer is Stoke's Law.
@@inspire191 Isn't stoke's law related to viscosity?
@@srirampatnaik9164 yes it is..and then there is another law by Stokes which describes phase change experienced by waves in such conditions.
@@inspire191 Ah okay thanks
Sir is the velocity of water is more than that of oil
Let's call the reflective index as n.
If n of oil is 1.1, n of water is 1.333,
Light travels from a less dense medium, to denser medium will slows down.
Just like moving your hand in air, compared to moving your hand in water.
Whichever medium is denser, will slow down the speed of light
my textbook was literally useless in explaining this thx!
God bless you sir ❤️
Suggested by our buet topper Meftaul Alam Siyam.I'll show my comment to him when I meet him In Shah Allah
Make one on Newton's rings
Also phase difference is half of lambda.. how???
x = 2nt cos r - ( 1/2 lambda)
I dont understand how exactly thin film filters certain wavelenghts , is it bc of phase shift? It it bc some wavelenghts get constructive int whan reflecting and destructive whan refracting?
Thanks mate!!
Just a note - seeing how you observe the ray in this scenario, it looks like they rray must be coming directly from the eye. Are we talking about emissive eyes? Otherwise the delta x would not be 2t, but instead 2t/cos(a) where a is the angle of incidence of light. Oh, right, my assumption is not accurate either since the light would change direction as being refracted, thus also changing path length.
Thank you.
What is to comment on the ray of light reflecting from the bottom surface in the water?
why does light shift 180° qhen reflecting when it reflects??
like... what makes the light shift that way? (be as technical as you want! using obscure words will just give me more to learn XD)
Look light shifts when going from a region of low refractive index to high refractive index because of the change in velocity because these two are inversely proportional to each other so when it moves to a region of high index it's velocity changes and when it's velocity decreases it bends from it's original path because of which if we compare these two the angle will shift
Oil has a refractive index higher than water, if so how can light travel slower in water . there must have been no phase difference
So basically the wavelength is no longer considered as coherent sources as one is being shifted by pi?
you saved my life sir hehe ^^
Yes he is still saving 😉😂😂😂😂😂😂
Yes he is still saving 😉😂😂😂😂😂😂
Low to high, phase shift of Pi.
2t is only accurate is you are considering only 90 degree interactions.
Nice video
what is the scope of physics?
You are real Mexican Gangster... May Jesus bless You
Physics 130 exam? Lol
@@danieltroconis6805 See who I found.....lol
Sir, what are the factors affecting the thin film and how to protect the thin film?
Stupid comment
Except oil is slower than water! So there wouldn't be any phase shift at the second interface.
Jai Shree Raam
Best explanation on whole of you tube.
Dear khanacademy: If "n" can be any value why the film needs to be "thin"? That is, if the film is "fat" why will not be any interference? What is the limit value for "n" that suports an interference pattern? Thank you.
You may have probably got the answer, anyway let me explain something I read earlier.
That when the film is much thicker than the visible light wavelengths, the effect permits nearly all of the spectrum to be seen and the resultant is Patten less and kinda whitish. On the other hand when the film is more thinner only certain colors are permitted and the result is usually one color or a mix of two till three colors which is more intresting and colorful pattern,used in various things.
Dear Sara: Thank for you answer. Just one more question: in a bubble or oil layer you usualy can see almost all the visible colours from red to violet, so that means the film is enough "fat" to allow seen of all visible spectrum; that is all colours are permited and we still can see the interference (no whitish). I know in the practice if the film is fat you can not see the interference pattern, but still can´t understand why and I asume that an ecution can demonstrate that, could you please give your avove explanation in numbers or formulaes as an example I can understand? Best regrads
Delta X for oil would be n times 2t, where n is the refractive index.
We consider optical path length not normal length
thankyou!
Shouldn't the path difference be optical path difference and should be 2mu(t).
Here the vacuum really looks like space vacuum- *Totally black* 😊...
How is a light ray partly reflected and partly refracted?
awesome!!!
MISTAKE...... !
actually the total path difference between the the ray reflected at the first surface and the ray reflected at the second surface should be = 2( refractive index of first medium )x(thickness of first film)
at 2:49 , how can wave 1 and 2 meet to interfere if they are parallel
This is drawn in 1D, think about the waves as 2D as in young's double slit and then you would understand how they interfere.
thanks!!! :))
how does interference occur when sun light isn't even a monochromatic light ?
Different colours show constructive and destructive interference on the basis of their wavelength at any given point . Read Concepts of Physics part 1 by HC Verma : chapter Light Waves if you havent already
you noticed he draw a triangle and called it an eye
Take a closer look
Syed Ifraz hevuh
Why no f body explained the pi shift before, bro here did it perfectly
i guess delta x=2ut where u is refractive index of oil
Thanks you very much!!!
holy shit thank you
I am from lraq... Thank you❤️❤️❤️❤️
Sorry...What is 'pie' shift?
180 degree displacement in the x axis
Sinling Ang... That simply means that the wave changes phase
it means that the waves are lagging 180 degrees behind the other.
It's destruction a case in interference think I replayed you soon lol
Goat
Why is it happening? Apart from rule.. :/
But speed of light in water isn't slower than oil thats majorly confusing. Y u do this.
They literally mentioned the value of the speed of light in oil why do u think it ain't slower?
Today attendance
No angel no reflection 😅
OMYGOSH WHAT DO I WORRY ABOUT
The equation should be 2dn = m(lambda)
I am so lost
sir ... i think u πshift it wrong..sorry...but..am i right?
too dificult
you should have chosen a better diagram
Nak ke nak me bat kr rhe kya🙄
Very bad diagram
Tysm sir
Thank you.