Use of Interference to Extinguish Reflection of Light from Glass

The phenomena of interference of light rays reflected from the two boundaries of a thin film cause the intensity IR of the resultant light to be greater or less than the intensity of the individual rays. The value of IR is determined by (a) the refractive index of the film, n₁, and the substance on which the film is deposited, ng, (b) the thickness of the film, (c) the angle of incidence at which light strikes the film, (d) the wave-length of the light. The conditions necessary to obtain extinction of reflected light are described. In the case of monochromatic perpendicular light IR=0 when n₁=ng1/2 and the thickness t is given by t=0.25λ(2a+1)2 where a is zero or an integer. The technique of building films which have the required values of n₁ and t is described. The films are made by building up monomolecular layers, each layer consisting of 50 percent cadmium arachidate and 50 percent arachidic acid. The film is then soaked in alcohol which dissolves the arachidic acid and leaves a skeleton of cadmium arachidate. The refractive index of the skeleton is determined by the amount of arachidic acid which has been removed from the film. Since the interstices are of molecular size there is no appreciable scattering of the reflected light.