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

Lesson 5: Diffraction and interference of light

# Thin film interference (part 1)

Thin film interference occurs when light waves reflecting off the top and bottom surfaces of a thin film interfere with one another. This type of interference is the reason that thin films, such as oil or soap bubbles, form colorful patterns. Created by David SantoPietro.

## Want to join the conversation?

• So since the velocity of light in a medium will be v = c/n..can we say that light reflected off a substance with a higher index of refraction will have a pi shift?
• That is correct!
(1 vote)
• At , oil has a higher index of refraction than water, so the velocity of light in oil should be slower than in water, and therefore there wouldn't be a pi shift when light reflects off the oil-water boundary.
• well, he did say that he was just making up a number and so he used the wrong value
• Why there is no pi shift when the light moves from a slow to fast medium? If a light ray moves from a higher refractive index medium to lower refractive index medium, why there is no pi shift?
• At you say that there may be a pi shift. What actually determines if there will be one?
• How do you determine if something is "fast" or something is "slow"?
• Well, the quality of a medium that determines whether light travels quickly or slowly is called the index of refraction, which is very important in this formula: v= c/n, where v is the speed of light in the medium, c is the speed of light in a vacuum(3x10^8 m/s), and n is the index of refraction. As you can see, the smaller the index of refraction, the faster the speed. What they were specifically talking about in this video were relative speeds: how fast light travels in one medium compared to how fast light travels in another medium. The narrator was talking about how fast light travels in a medium compared to the medium around it. For example, glass might be a "fast" medium compared to diamond, because light travels faster in glass than in diamond. However, glass might be a "slow" medium compared to water, because light travels faster in water than in glass.
• Hi, I think I understand that when light travels in a medium that lowers its velocity, it experiences a change in phase relative to its original phase. My question is why does that phase difference have to be pi? Intuitively, I would assume the change in phase is proportional to the change in velocity. Thanks!
• There is no change of phase due to change of speed. There is a change of phase when light reflects at a boundary where it is going from a fast medium to a slow medium; the reflected light is shifted by pi.
• Every time light reflects off of a slow substance, there's a pi shift.WHY?
• When the light ray moves from air to the layer of oil at isn't it supposed to be refracted instead of it traveling in a straight line like that? Oil has a higher refractive index than air