Refractive index
When light passes from air or a vacuum into a transparent crystal, some of it is reflected at the surface and some refracted. The latter is a bending of the light path that occurs because the speed of light changes as it goes into the crystal; Snell's law relates the bending angle to the Refractive index, the ratio of speed in a vacuum to speed in the crystal. Crystals whose point symmetry group falls in the cubic system are "isotropic": the index does not depend on direction. All other crystals are "anisotropic": light passing through them is broken up into two plane polarized rays that travel at different speeds and refract at different angles.