Interference in Thin Metallic Films

The phenomenon of the colored appearance of thin metallic films sputtered upon glass or upon other metals, when viewed in white light, is explained as being caused by interference. Some qualitative observations of cyclic changes of color are given which support this viewpoint. Mathematical expressions are derived from Maxwell's electromagnetic equations to express the change of phase as the wave train is reflected from a metal surface in air, from metal in metal, for refraction as the wave enters the metal from air and from air into metal. Use of the values of the indices of refraction and absorption for massive metals indicates fair substantiation of experimental results. Fritze's values of the optical constants for thin metals do not seem to give agreement between theory and experiment. The thickness of the film of copper necessary to produce a yellow color (interference for $\lambda =0.48\mathrm{}\mu$) when calculated from the index of refraction (1.13) is found to be three and one-half times larger than the value found by weighing the total deposit. The balance of evidence seems to support the theory of interference.