The speed and wavelength of a wave depends on the medium through which the wave travels. Therefore, when a wave approaches a boundary obliquely (at an angle), their direction is changed, because of the change in the speed of the wave. This change in the direction of waves at the boundary between two different media is known as refraction. During refraction, the speed and wavelength of the wave changes, but the period and frequency of the wave remain the same.

When you look at a pencil that emerges from water it looks like it is bent. This is because the light from below the surface of the water bends when it leaves the water. Your eyes project the light back in a straight line and so the object looks like it is a different place.

Key terms

Refraction: The bending of a wave in moving from one medium to another of different density.