Waves in weakly anisotropic 3D inhomogeneous media: quasi-isotropic approximation of geometrical optics

The quasi-isotropic approximation (QIA) of geometrical optics is outlined. The main idea of the method is that electromagnetic waves in weakly anisotropic media preserve their transverse structure as they do in isotropic media. Advantages of the QIA are illustrated by considering electromagnetic wave propagation in plasma, a number of optical problems (liquid crystals, hiral media, single mode optical fibres), acoustical problems of weakly anisotropic elastic media, and quantum mechanical polarisation effects of the Stern—Gerlach type. New modifications of the QIA are presented, namely the method of split rays and the synthetic approach, the latter being applicable even for strongly anisotropic media.