Electromagnetic waves in unsteady anisotropic media

For an anisotropic medium whose dielectric tensor is a known function of time, an evolution operator is found, expressed by a multiplicative integral, characterizing the change of the electromagnetic field of a wave with given wave vector. The tensor of the instantaneous wave resistance is introduced, and its relation to the dielectric tensor and the wave normal is studied. The tensor of the instantaneous wave resistance is used in seeking approxmate solutions for media with slowly varying properties.