Refractive index varying through time.
Refractive index is a measure of how much light is bent when passing from one medium to another. A familiar example is how a stick looks bent when it enters water. Refractive index is calculated as the speed of light in a vacuum divided by the speed of light in the material. Light travels fastest in a vacuum and always slows in other media. It slows very slightly in air more slowly in water, slower still in glass and slower again in diamond. This animation shows the change in appearance of a stick passing through a blue block of material whose refractive index varies from 1 (vacuum or approximately air) to over 2.4 (diamond).