ABSTRACT
When embarking on the work of this book the authors did not realize fully how multi-faceted the applications of the quasi-isotropic approach are. It applies to vector fields of an arbitrary physical nature: electromagnetic, elastic, spinor. In essence, the QIA concludes the last chapter in the geometrical optics of inhomogeneous media creating the basis for the geometrical optics of weakly anisotropic media. For a long time this branch of geometrical optics was simply absent and there was even a prejudice that geometrical optics has no right to exist in the limit of weak anisotropy. Only with the advent of the QIA has it become clear that weak anisotropy is not an absolute impediment to using the ray method and that the prohibition from applying geometrical optics to weakly anisotropic media only applies to the approaches known previously, i. e., the Rytov method (isotropic media) and the Courant-Lax method (strongly anisotropic media). As a result, a new branch of the ray method has emerged — the geometrical optics of weakly anisotropic media. This branch has already given rise to abundant results and linked together the old branches of geometrical optics that previously existed on their own.
