Some mechanisms of formation of laser-induced periodic surface and bulk structures in condensed matter are considered. Their influence on optical properties of semiconductors is investigated. It is shown that under action of coherent laser beams (CLBs) a grating of the refraction index arises in semiconductors. The models featuring this effect are offered. The relevant theoretical calculations confirming occurrence the new phenomenon - appearance of a grating of the refractive index in semiconductors in a field of CLBs - were carried out. It is shown that in ferromagnetic semiconductors (FMSC) with a laser-induced grating of the magnon temperature one probably deals with occurrence of the nonlinearity of the refraction index caused by the nonlinearity of FMSC magnetic susceptibility and one should observe oscillations of a refraction index, which will result in occurrence of complementary darkened areas. (C) 2004 Elsevier B.V. All rights reserved.