5.4 Wave Properties: Refraction
Waves change speed when they cross a boundary between different substances. Refraction is a property of all waves. The change in speed of the waves causes a change in direction. When light enters a more dense substance, such as going from air to glass, it slows down and the ray changes direction towards the normal. However when light enters a less dense substance, such as going from glass to air, it speeds up and the ray changes direction away from the normal. However if the ray is travelling along the normal it will not change.