What is the name of the index that describes how much light is bent when entering a material?
In optics, the refractive index or index of refraction of a material is a dimensionless number n that describes how light propagates through that medium. It is defined as n = c/v where c is the speed of light in vacuum and v is the phase velocity of light in the medium. For example, the refractive index of water is 1.333, meaning that light travels 1.333 times faster in a vacuum than it does in water.
The picture illustrates the refraction of light through a plastic block.