LIGHT TRAJECTORY IN GEOMETRICAL-OPTICS AND METRIC OPTICS

The light trajectory in an inhomogeneous medium is studied by the vari ation of Lagrangians L and L which correspond to Fermat's principle in the geometrical optics and the null geodesic in the metric optics, re spectively. The relation between the metric coefficients of the three- dimensional space and of the four-dimensional space-time is establishe d. The physical meaning of the equivalence and difference of both the descriptions is revealed. It is shown that Fermat's principle is a dir ect result of the null geodesic.