Light can vibrate in one plane (be polarized) through selective absorption or reflection. When light passes from one medium to another of different density, some of the light will be reflected and some will pass through but be bent and slowed down (refracted). The angle of reflection equals the angle of incidence, while the angle of refraction depends on the change in velocity. The index of refraction and relief of a mineral are used to identify it. Index of refraction is a measure of how light slows down when passing into a mineral and is related to its crystal structure. Relief refers to the difference in index of refraction between a mineral and its surroundings, and is used to estimate the index of ref