To ensure almost 100 per cent transmitivity, photographic lenses are often coated with a thin layer of dielectric material. The refracive index of this material is intermediatead between that of air and glass (which makes the optical element of the lens). A typically used dielectric film is `M_gF_2` (n=1.38). What should the thickness of the film be so that at the centre of the visible spectrum `(5500 oversetcircA)` there is maximum transmission.