To ensure almost 100 per cent transitivity, photographic lenses are often coated with a thin layer of dielectric material. The refractive index of this material is intermediated between that of (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 center of the visible spectrum (5500 Å) there is a maximum transmission
IA is an incident ray at point A is such that the incident angle I is formed from air to the film surface.
AR1 and AD are the reflected and refracted rays, respectively. D is the point on which the partial reflection of glass and film interface. CR2 and AR1 are parallel.