If a wave is incident on the boundary between two media at an angle to the boundary, that part of the wave which is transmitted will change direction. It will be refracted. Any wave can be refracted – longitudinal (sound, pressure waves) or transverse (electromagnetic).
The diagram above shows light incident on a glass block. As light enters the block it is refracted towards the normal. At the other end of the block where it exits, it is refracted away from the normal. With the angles labelled as above, the following relationship between the angle of incidence
the refractive index
the angle of refraction
and the refractive index
of the refractive medium is given by Snell's Law:
The refractive index of the vacuum is 1. The refractive index of air is very nearly 1 so we can write
The reason for the change of direction of the wave is that waves travel at different speed in different media. In general light waves travel slower in denser media, while sound travels faster in denser media. For light waves in different media 1 and 2 the following relationships between the speed, frequency and wavelength exist:
Notice that the wave equation
applies in both media:
