When a light wave passes from a rarer medium to a denser medium, there will be a phase change of `pi` radians. This difference brings change in the conditions for constructive and destructive interference. This phenomena also reasons the fromation of interference pattern in thin films like, oily layer, soap film, etc., but has no reason on the shifting of fringes from the central portion outward. The shift is dependent on the refractive index of the material as per the relation, `Delta y = (mu - 1) t`
Thin film interference happens with

Options

A. point or spherical source
B. broad source
C. film thickness of the order of `10,000 Å`
D. very thick transparent slabs

Answer

b., c

Explanation

Broad sources provide wide angular incidence of ligth. The thickness should be small, since the path difference should be comparable with the wavelength. Thick slabs cannot bring wavelength comparable path difference. So, choices (a) and (d) are wrong. Choices (b) and (c ) are correct.