Subject: Applied Physics 2

Topic: Interference And Diffraction Of Light

Difficulty: Low

Highly reflecting film is used in all kind of sun control films used on car, sun glasses etc. when light is incident on the glass surface. Part of it is reflected and part of it is transmitted. For highly reflecting films, majority of light needed to be reflected and this can be achieved if the two reflecting rays would interfere constructively .

For constructive interference, OPD Δ = nλ

i.e 2$μ_f$ t cos r ± λ/2 ± λ/2 = nλ

since the reflection at B and D both takes place at a surface of denser medium.

2$μ_f$ t cos r ±λ= nλ

2$μ_f$ t cos r = (n± 1) λ

For normal incidence cosr =1

Let t= $t_ {min}$ for n= 0

Therefore, 2$μ_f$ $t_ {min}$ = λ

$t_{min}$ = λ/ 2$μ_f$

For a highly reflecting film the minimum thickness of the film should be λ/ 2$μ_f$