Abstract
A simple method of determining the effective mass m* of free electrons in thin metal films (or the film thickness) only from the measured transmission of light in the range of the plasma reflection is discussed. The method is verified for gold films with thicknesses ranging from 7-40 nm. It was stated that the effective mass in this material does not depend on the film thickness and equals (1.0+or-0.03) me. On the other hand, knowing the value of m*, the absolute film thickness can be obtained directly with a high accuracy within an error of a few angstroms, which is better than by the alpha -step or the Tolansky method.