A material having a refractive index of 1.25 is used to coat a piece of glass (n=1.55). what should be the minimum thickness of this film to minimize reflection of 550nm light.

The minimum thickness of the film is 1.1*10^-7 m.

Explanation:

let Δx be the thickness of the coating.

let n be the refractive index.

let λ be the wavelength of the light.

then, the relationship between the thickness of the coating and different parts making up the functioning of the film is:

Δd + Ф = [m + 1/2]λfilm

where, λfilm = λ/n

Δd + Ф = [m + 1/2]λ/n

but for the minimum thickness, m = 0, because the rays of the light are reflected from a low index material / medium to higher index material. medium, Φ = 0 and Δd = 2Δx

that is:

2Δx = [1/2]λ/n

Δx = [1/4]λ/n

= 1/4*[ (550*10^-9) / 1.25]

= 1.1*10^-7 m

Therefore, the minimum thickness of the film is 1.1*10^-7 m.