Question

To ensure almost 100 per cent transmittivity, 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 center of the visible speetrum (5500 Armstong) there
is maximum transmission.

Answer 1

It is a matter of destructive interferences for the refracted waves.

(By the way, it is 5500 ångströms) http://en.wikipedia.org/wiki/Angstrom