When light passes from one medium (material) to another it changes speed. This is because the speed of a wave is determined by the medium through which it is passing.
When light speeds up as it passes from one material to another, the angle of refraction is bigger than the angle of incidence.
For example, this happens when light passes from water to air or from glass to water.
The diagram above shows light incident on a water-air interface.
When the angle of refraction is equal to , the angle of incidence is called the critical angle, .
The angle of refraction cannot be greater than . Look at the two images below to see what happens as the angle of incidence increases.
Angle of incidence less than critical angle
At any angle of incidence greater than the critical angle, the light cannot pass through the surface - it is all reflected.
This is called total internal reflection.
The relationship between critical angle, , and refractive index, is .
Calculate the critical angle for red light incident on a water-air interface.
The refractive index of water is for this colour of light.
Critical angle of water for this light