Dispersion occurs when different frequencies of light have different phase velocities, due either to material properties ("material dispersion") or to the geometry of an optical waveguide ("waveguide dispersion"). The most familiar form of dispersion is a decrease in index of refraction with increasing wavelength, which is seen in most transparent materials. This is called "normal dispersion". It occurs in all dielectric materials, in wavelength ranges where the material does not absorb light. In wavelength ranges where a medium has significant absorption, the index of refraction can increase with wavelength. This is called "anomalous dispersion".