Light from directly behind you reflects off the top surface of a thin soap film

Question

Light from directly behind you reflects off the top surface of a thin soap film floating on water. Assume that the incident light is exactly normal to the surface of the film and that the index of refraction of soap is between that of air and water.

a) If there is a pattern of light and dark fringes in the soap film, explain from where the interfering waves are coming from.

b) Assume that the soap has n= 1.25. If there is a bright red fringe-- wavelength = 700 nm- observed normal to the air/soap interface, then the minimum possible thickness of the soap film must be (choose one)

125 nm

560 nm

700 nm

100 nm

350 nm

280 nm

Explanation / Answer

a) from reflecting off the air soap interface and the soap water interface

b) 2 d = lambda/n

d = lambda/(2n) = 700/(2*1.25)=280 nm

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