index of refraction

A material's index of refraction is defined as the speed that light travels in the material divided by the speed of light in a vacuum. Therefore, the index of refraction of the vacuum is equal to one. Light slows down as it enters a material due to the interaction between the oscillating electric and magnetic fields of the light wave and the constituent parts of the material. The index of refraction of air is 1.003, and the index of refraction of water is 1.33.