When we hold a stick partly in the water, it looks as though the stick bends just where it enters the water. That is due to the change of the direction of the light after it enters the water. This change in the direction of the light rays is called refraction. Glass, water and other solids and fluids each have different powers of refraction.

The law of refraction comes into operation when a ray of light passes through a smooth surface bounding two media not homogeneous, such as air and water, or when rays traverse a medium the density of which is not uniform, such as the atmosphere.