Question

If a light of wavelength $\lambda$ is incident normally and the thickness of film is $t$, then optical path difference between waves reflected from upper and lower surface of the film is

• A. ${2n}_{1}t$
• B. ${2n}_{1}t-\frac{\lambda }{2}$
• C. ${2n}_{1}t+\frac{\lambda }{2}$
• D. $2t$

Answer 1

The correct option is A ${2n}_{1}t$

Some part of the light gets reflected from the coating surface as light 1 and the other part gets refracted which further gets reflected from glass and after refraction ,emerges as light 2 and interfere with light 1.

Optical path the light 2 travelled from A to B and then From B' to A' be $x$.

Let $t$ be the thickness of the film.

Then $x=2\times n_{1}t$

Thus the total path difference is $x=2n_{1}t$