The refractive index of any material depends on the wavelength of light because different wavelengths are interfered with to different extents by the atoms that make up the material. In general refractive index varies linearly with wavelength. Materials can be divided into 2 classes based on how the velocity of light of a particular wavelength varies in the material. Materials whose refractive index not depend on the direction that the light travels are called isotropic materials. In these materials the velocity of light does not depend on the direction that the light travels.
Isotropic materials have a single, constant refractive index for each wavelength.
Minerals that crystallize in the isometric system, by virtue of their symmetry, are isotropic. Similarly, glass, gasses, most liquids and amorphous solids are isotropic. Materials whose refractive index does depend on the direction that the light travels are called anisotropic materials. These types of materials will have a range of refractive indices between two extreme values for each wavelength. Anisotropic materials can be further divided into two subclasses, although the reasoning behind these subdivisions will become clear in a later lecture.