Question

What should be the minimum thickness of this film?
A material having an index of refraction of 1.28 is used to coat a piece of glass, n = 1.50.
What should be the minimum thickness of this film if it is to minimize reflection of 480 nm light at normal incidence?
What about the material's index of refraction?

Answer 1

Answer:

$d=93.75\,nm$ is the minimum thickness of the coating of given material.

Explanation:

This method of application of non reflective coatings uses the phenomenon of destructive interference in which the the waves reflected from the top surface of the coating are out of phase to the waves reflected from the bottom layer of the coating.

Given that:

refractive index of coating, $n_c=1.28$

refractive index of glass, $n_g=1.5$

wavelength of light, $\lambda=480\times 10^{-9}\,m$

(Here we observe that refractive index of air<refractive index of coating<refractive index of glass)

We have the mathematical relation for film thickness 'd' as:

$d=\frac{\lambda}{4.n_c}$

$d=\frac{480\times 10^{-9}}{4\times 1.28}$

$d=93.75\,nm$