Question

A glass plate (n = 1.59) is covered with a thin, uniform layer of oil (n = 1.25). A light beam of variable wavelength from air is incident normally on the oil surface. Observation of the reflected beam shows constructiveinterference at 630 nm. Determine the minimum non-zero thickness of the oil film.
(in nm)

Answer 1

when reflection takes place on top surface of oil, 180o phase difference or wavelength difference /2 occurs, because reflecting surface is denser medium

Reflected rat from glass surface also gets 180o phase difference or wavelength difference /2 , because reflecting surface is denser medium.

path length in oil for the reflected ray from glass surface = n 2t , where n is refractive index of oil and t is thickness of oil film.

for constructive interference, n 2t =

thickness of oil film to get constructive interference = / (2 n ) = 630 nm / ( 21.25) = 252 nm