A thin soap film ( n = 1.32) suspended in airhas a uniform thickness. When white

Question

A thin soap film (n = 1.32) suspended in airhas a uniform thickness. When white light strikes the film atnormal incidence, violet light (?V = 418 nm) is constructively reflected. At what angle of incidence will greenlight (?G = 562 nm) be most strongly reflected?Assume the film has the minimum thickness that can produce these reflections?

40 degrees is not the correct answer. Someone already gavethat answer & it was wrong so if anyone else could figure itout that would be great. I always rate!

Explanation / Answer

The 1st-surface reflection is phase-reversed and the 2nd-surface reflection is not. Thus for reinforcement, the two-way trip through the mica is an odd-integer multiple of 1/2 wavelength lambda, and the minimum thickness T(minimum) = lambda/4, where lambda is the wavelength in the soap film = 418/1.32 nm.
A. Since T(minimum) is proportional to lambda, it must increase if lambda does.
B. T(minimum) = (560/1.33)/4 = 105.263 nm

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