Question

A lens is coated with a thin film of transparent substance magnesium fluoride (MgF2) with n = 1.38 to reduce the reflection from the glass surface (n = 1.50). How thick a coating is needed to produce a minimum reflection at the centre of the visible spectrum (λ = 550 nm)?

Answer 1

We assume that the light strikes the lens at near-normal incidence. We would like to find the thickness of the film that will bring about destructive interference between rays R1 and R2 (Fig. 12.9). Phase change of π is now associated with each ray and the condition of minimum intensity is now.

Thus, the thickness of the thin nest possible film is (for m = 0)