Refraction is a bending that occurs when a wave moves from one medium to another at an angle and changes speed.

Refraction is a bending that occur when wave moves from one medium to another, each being of a different density. It is the change in density of the medium which causes refraction or the bending of light to occur. If light is moving from a less dense medium to a more dense medium, then the angle of refraction will be less than angle of incidence, meaning that light will bend and move at a slower speed. On the other hand, the opposite will happen when light or other wave travels from a more dense medium to a less dense medium.