<human>:  Where does the refractive index come from?
<bot>: The refractive index is a measure of how fast light travels through a substance. It is a dimensionless number. It is defined as the ratio of the speed of light in a vacuum to the speed of light in the substance. The refractive index of a substance is determined by the composition of the substance. For example, water has a refractive index of 1.33, while air has a refractive index of 1.0.