A piece of glass has a thin film of gasoline floating on it. A beam of light is shining perpendicular on the film. If the wavelength of light incident on the film is 560 nm and the indices of refraction of gasoline and glass are 1.40 and 1.50, respectively, what is the minimum thickness of the film to see a bright reflection?
A) 500 nm
B) 400 nm
C) 300 nm
D) 200 nm
E) 100 nm

Question

Quizplus · Accepted Answer

For constructive interference (bright reflection), the minimum thickness t of the film is given by t = (m + 0.5) * (λ / (2 * n)), where m = 0 for minimum thickness, λ is the wavelength in the film, and n is the refractive index of the film. Here, λ in the film = 560 nm / 1.40 = 400 nm. Thus, t = (0 + 0.5) * (400 nm / 2) = 100 nm.

A Piece of Glass Has a Thin Film of Gasoline