The influence on the optical reflectivity and the complex index of refraction of thin conducting films, due to the variation of thickness and electronic mean free path, is investigated. The calculation is based upon Chambers' kinetic analysis. It is shown that, when the penetration depth exceeds the film thickness, with diffuse scattering of electrons at the boundaries, the resistivity is an oscillatory function of both frequency and thickness. Consequently, these oscillations will result in fluctuations in the optical reflectivity. The "optical-size effect," which is important in conducting films of thickness <~10-5 cm, may give rise to pronounced changes in the characteristics of the frequency variation of the optical reflectivity.