ABSTRACT
Geometrical optics at the elementary level is usually presented as a closed theory based on the concepts of phase, rays and amplitude propagation along the rays. It is also usually stated that such a theory describes well short wave radiation and in this form the subject is fairly old. In order to focus on the fundamental physical and mathematical ideas, the treatment will be heuristic. Whereas for a plane harmonic wave the wave number and frequency are constant, for a general wave train in the form a Fourier integral, the concepts of wave number and frequency are only local in the sense that they depend on space and time.
