Refraction is the bending of light as it passes from one medium to another, such as from air into water. This change in speed causes the light to change direction, which is why objects underwater appear distorted or shifted from their actual position. The degree of refraction depends on the materials involved and the angle at which the light hits the surface. For example, when light enters a denser medium like glass, it slows down and bends towards the normal line, while it bends away when exiting into a less dense medium.