The problem of the propagation of electromagnetic waves through solids is essentially one of interaction between light quanta and matter. The most fundamental and general treatment of this subject is therefore undoubtedly based on the quantummechanical theory of this interaction. Nevertheless, a discussion in terms of a classical field theory often is quite useful. These more restricted theories are as a rule easier to handle than the general theory. Moreover the properties of matter wich do occur in classical theory are often more amenable to direct measurement than the matrix elements needed for evaluation of the results of quantum theory.