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### Course: AP®︎/College Physics 2>Unit 6

Lesson 5: Diffraction and interference of light

# Thin film interference (part 2)

Let's work out a few details on how thin film interference works as we explore the concept of thin film interference as it relates to light waves. Understand how light's wavelength changes as it moves through different mediums, like air and oil. Learn how to calculate the wavelength in a new medium using either the speeds of light or the indices of refraction. Master the conditions for constructive and destructive interference. Created by David SantoPietro.

## Want to join the conversation?

• At it starts to explain m but I am still at a loss as to what m signifies. How would we know what to plug into "m" when reading a word problem?
• You don't really need m here, it is just that if the thickness of the thin film you get is ANY integer times the wavelength, then the interference will be constructive, and if the thickness of the thin film is any integer + a half times the wavelength, then the interference will be destructive. Of course again this will be the other way around if you have the case of one of the two waves being phi shifted. If you are reading a word problem, you would be given the thickness of the film. You are not able to find it out by multiplying the wavelength by some "m"-value, unless it is stated in the word problem for example that the thickness of the film is two times the wavelength, in which case you wouldn't even need to know the thickness because you know how thick it is in relation to the wavelength, and that is all you need to know to find out whether the interference will be destructive of constructive.
• What if the rays are not perpendicular to a surface? Won't there be more to the path difference ?
• Excellent question, and yes you are right, the path difference does get larger. That's why if you look at thin film interference head on and then look at it from the side, you will see the interference pattern shift in a way that is equivalent to the film getting thicker.
• In my physics book for thin film interference, constructive interference says the equation is 2t = (m + .5) lamda(n) why do you have them opposite??
• Hi, in the video, David said that if light goes form a medium that have large v into a different medium have a small v then the light wave will have a pi shift, and so the equation for the const and dest will be switched. Maybe this is the case in your textbook.
• Its not clear to me what "m" stands for here. It usually means the order of the maxima, but how does that apply here?
• Consider the equation 2t = m/\ ( /\ = wavelength) ,which is applicable for constructive interference if there no or n/\ times change in the path difference. Now by rearranging the equation, we get "t = m/\ / 2" . So, as it is clearly seen now , "m" is merely a parameter which determines whether constructive or destructive interference will take place for the thickness "t".
• I think I get it all apart from the fact that you don't give us when there is a pi shift. Is it when the third medium is optically denser than the second one?
• A pi shift is when you have at least 2 substances and the 2nd medium is slower, or as you put it, denser.
• At about , why are you multiplying by λA instead of λB? I thought we were supposed to multiply by the wavelength within the film?
• If you have lambdaB then by all means, use that. The formula then is 2t=m*lambdaB. However, usually you are only given the wavelength in air. The (vB/vA)lambdaA is how you convert lambdaA into lambdaB.
• Why isn't a definite pattern observed in oil thin film interference (as seen is double slit interference)?
• I bet you can figure that out for yourself if you think about it a little bit. Compare what's happening to the light rays in double slit to what's happening in thin film.