Refractive Index
Every medium has a specific refractive index - this is a value which describes how fast light travels through the medium. For example, an object with a refractive index of 2 will let light pass through it at 1⁄2 the speed it travels through a vacuum.
The higher the refractive index of a material, the more it slows down, and the more it 'bends' towards the normal.
The equation for refractive index is: n = c⁄v, where n is the refractive index, c is the speed of light through a vacuum, and v is the speed of light through the medium.