I understand that thin oil is color because of constructive/destructive interference, but why isn't the same true for thick oil? Is it because the first ray that reflects off the oil reaches your eye before the second ray that refracts through the oil and reflects off of the water?
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2 Answers
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In essence, what you have is a fabry perot etallon. The number of resonant frequencies are D=n*lambda/2 (n integer) with D the thickness of your oil. When D is thick, there are a lot of lambda's that will constructively interfer and so you won't see distinct colors (peaks will be too close for your eye to discern)
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Thin film surfaces vary and are between 1 and 1000 nm thick. The wavelength of visible light is around 300 to 800 nm. There is not enough distance for constructive or destructive interference. When a photons wavelength matches the thickness then that's the color you get in that area and other colors are eliminated. Sometimes you get two or more colors that match that thickness and they mix in that area. A lot of thin film surfaces are purple and pink looking. Some thin films completely eliminate certain colors or UV rays for instance.
answered Dec 18, 2015 at 1:29