If we consider the medium through which electromagnetic waves propagate to comprise of atoms whose electrons are driven in an oscillatory manner by the electromagnetic field, then the resulting equation for the index of refraction of the medium is dependent upon the frequency of the electromagnetic radiation, the number of electrons, the natural frequency of the electrons and their damping factors. In this general case, the index of refraction is a complex quantity. The real component of the index of refraction is related to the velocity of the electromagnetic wave through the medium as n = c / v, where c is the velocity of electromagnetic radiation in a vacuum and v is the velocity in the medium.