Further generalization of Fermat's principle

From Maxwell's equations for electromagnetic fields, time-averaged energy n ow density vector of stable monochromatic linearly polarized light in an is otropic insulative nonmagnetic medium is deduced. By the introduction of ti me-averaged energy new density rays and the definition of new generalized r efractive index n(G1), Fermat's principle of geometric optics is further ge neralized and its application conditions are discussed. The generalized Fer mat's principle can be used to describe stable transmission of light in a m edium with variable refractive index. The necessary and sufficient conditio ns of a nondivergent and nonfocusing light beam are derived from this Ferma t's principle.