In a thin film interference experiment, a film of oil (n=1.45) floats on water (n=1.33). If the film appears green (λ=550 nm) when viewed with light perpendicular to the surface, what is the minimum thickness of the oil film?

Core Concept

For constructive interference in a thin film with one phase reversal (air-oil interface), the condition is 2nt = (m + 1/2)λ, where n is the refractive index of the film, t is the thickness, and λ is the wavelength in vacuum. For the minimum thickness (m=0), we get t = λ/(4n) = 550 nm/(4 × 1.45) = 94.8 nm.