(a) A thin film of oil (n = 1.20) is spread over a puddle of water (n = 1.33). I

Question

(a) A thin film of oil (n = 1.20) is spread over a puddle of water (n = 1.33). In a region where the film looks red ( ? = 630 nm) in reflection when viewed from directly above the film, what is the minimum possible thickness of the film? (b) If the oil film is surrounded on both sides by air (like a soap bubble), then what would the minimum thickness of the film be for a region where the film looks red (? = 630 nm) in reflection when viewed from directly above the film? Provide a sketch of each situation.

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Explanation / Answer

a) two low to highs 1->1.2 and 1.2->1.33

so 2 d = m lambda/n
d = lambda/(2n) = 630/(2*1.2)=262.5 nm

b) now one low to high so 2 d = (m + 1/2) lambda/n

d = lambda/4n = 630/(4*1.2) = 131.25 nm

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