Chapter 19, Problem 34A

To determine

The interference pattern by a thin film.

Explanation of Solution

Introduction:

Thin film interference occurs when light wave reflected from the top surface of a film experience constructive or destructive superposition with the light wave reflected from the bottom surface.

A thin film has a higher refractive index than air. When light reflects from the top surface of this film it undergoes a phase shift of one half wave length. For destructive interference two waves must be out of phase by one half of the wavelength. Therefore, for the wave reflecting from the back of the film, to have phase difference of one half of the wavelength, the thickness of the film should be a multiple of one half of the wavelength of light. Hence, when these two reflected light meet, a dark band is produced.