A thin film of soap (n=1.34) coats a piece of flat glass (n=1.52). How thick is the film if it reflects 643-nm red light most strongly when illuminated normally by white light?
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The film having refractive index of 1.34 is coated on the surface of a glass. The refractive index of the glass is bigger than the film. This results interference on the surface of a film as in the picture with a path differences of .
The path difference of causes the interference pattern on the surface of the film
with changing thickness d. As in the picture below.