If we assume normal incidence,the minimum thickness of a soap film (n = 1.33)in air that gives constructive interference when viewed by reflected light of wavelength 400 nm is
A)225 nm
B)200 nm
C)100 nm
D)75 nm
E)57 nm

Question

Quizplus · Accepted Answer

For constructive interference in a thin film with normal incidence, the condition is 2nt = mλ, where n is the refractive index, t is the thickness, m is the order of interference (m=1 for minimum thickness), and λ is the wavelength. Solving for t gives t = λ/(2n) = 400 nm/(2*1.33) ≈ 75 nm.

If We Assume Normal Incidence,the Minimum Thickness of a Soap