The bending of a light ray at the boundary of two medium as the light ray propagates from a medium to another with difference optical density.

The speed and direction of light changes when in different medium

• \(n=\dfrac{\text{sin i}^0}{\text{sin r}^0}\), 

 (Snell's law), where \(\text{i}^0\) = incidence angle, \(\text{r}^0\) = refracted angle

• \(n=\dfrac{D}{d}\), where D = real depth, d = apparent depth

• \(n=\dfrac{c}{v}\), 

 where c = velocity of light in air (\(3\times10^8\text{ms}^{-1}\)), v = velocity of light in a medium

• \(n=\dfrac{1}{\text{sin c}^0}\), 

 where \(\text{c}^0\) = critical angle