The optical properties of crystals. The transmission of light by crystals differs from that by glass since the refractive index may depend on the direction of incidence of the light (crystals are not in general isotropic).

Uniaxial crystals (belonging to the tetragonal, hexagonal, and trigonal systems) have two principal refractive indices and display double refraction. Biaxial crystals (orthorhombic, monoclinic, triclinic) have three principal refractive indices.