A property of a material that changes the speed of light, computed as the ratio of the speed of light in a vacuum to the speed of light through the material. When light travels at an angle between two different materials, their refractive indices determine the angle of transmission (refraction) of the light beam. In general, the refractive index varies based on the frequency of the light as well, thus different colors of light travel at different speeds. High intensities also can change the refractive index.
The refractive index of a vacuum is 1.0, and air is a tiny fraction greater than 1.0. The higher the index, the slower the speed of light through the medium, because the speed through the material is the speed of light (c) over the refractive index (n), thus speed = c/n. Following are common refractive indices. See fiber optics glossary
Material Index (n)
Lithium niobate 2.25
Gallium arsenide 3.35
** = air is a tiny franction
greater than 1.0
When light travels at an angle between two materials, light bends according to their refractive indices. In order to reflect, light must be on the wider side of the critical angle.