Question

A material with an index of refraction of 1.24 is used to coat glass. What is the minimum thickness of the coating that will minimize the reflection of light with a wavelength of 6950 ˚A? The index of refraction of glass is 1.5. Answer in units of µm.

Answer 1

For minimum reflecction path difference between reflected lights from glass and the coating should be lambda/2

(half of its wavelength)

and in this it goes, strikes and comes back to give interference pattern

so thickness would be half of path difference

It means thickness would be lambda/4n

where lambda/n is the wavelength in that medium

As it passes through coat so we will use refractive index of material 1.24

t = lambda/4(1.24)

t = (6950 x 10^-10)/(4.96)

t = 0.14 x 10^-6 m

t = 0.14 um