To ensure almost 100 per cent transmissivity, photographic lenses are often coated with a thin layer of dielectric material. The refractive index of this material is
intermediated between that of air and glass (which makes the optical element of the lens). A typically used dielectric film is MgF2 (n = 1.38). What should the
thickness of the film be so that at the centre of the visible spectrum (5500 Ao) there is maximum transmission?
IA is incident at point A such that there is an incident angle I formed from air to film surface. AR1 and AD are the reflected and refracted rays. D is the point where the
partial reflection from the glass and film interface. CR2 and AR1 are parallel to each other.
μ(AD + CD) – AB
AD = AC = d/cos r
d tan r = AC/2
d = 1000 Ao