ABSTRACT
This chapter explains the restrictions and develop the theory of optical systems within them. In order to do this, it introduces a further concept, the geometrical wavefront, in addition to the ray, already used. The wavefronts with rays, the basic concepts of geometrical optics, and optical path length is the basic physical quantity. The chapter discusses both Snell's law and Fermat's principle as basic postulates of geometrical optics, although, they are both consequences of electromagnetic theory. It is convenient also to mention explicitly the other properties of rays and wavefronts in geometrical optics. These are rectilinear propagation in homogeneous media, reversibility of ray paths, non-interference of intersecting ray paths and the inverse square law of illumination for a point source. An entirely different approach to refraction is provided by Fermat's principle, which is a stationarity principle of the same kind as the principle of least action of Maupertuis.
