Refraction occurs at the boundary between two transparent substances. Consider refraction at the boundary between two substances where the light passes from a substance where speed of light is c1 and goes into a substance where the speed of light is c2:
| sin i sin r | = | speed of light in medium 1 speed of light in medium 2 |
| = | c1 c2 | |
| n1sinθ1 | = | n2sinθ2 |
| n | = | speed of light in a vacuum speed of light in air |