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Unformatted text preview: 27.3 Thin Film Interference ight waves can interfere in many situations. All we need is a difference in optical Light waves can interfere in many situations. All we need is a difference in optical path length. As an example, lets consider a thin film of oil or gasoline floating on the surface f water: of water: Part of a light ray gets reflected (1) from the surface of the film, and part gets refracted (2). Then the refracted ray reflects back off the film/water interface and heads back into the air toward our eye. Thus, two rays reach our eyes, and ray 2 has traveled farther than ray 1. Thus, there is a difference in the optical path length . If the film is thin, and the ray strikes almost perpendicularly to the film, then the OPD is just twice the film thickness, or . 2 t l = Thus, if , we have CI and the m t = 2 film appears bright. If , we have DI and the film appears dark. ) ( 2 2 1 + = m t The optical path difference occurs inside the film, so the index of refraction that is important here is n film . hat is the wavelength of the light in the film ( lm What is the wavelength of the light in the film ( film )? vac film f c c n = = = vac film = film film film v f v film n ne more important point: One more important point: When waves reflect from a boundary, it is possible for them to change their phase.When waves reflect from a boundary, it is possible for them to change their phase....
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