Light waves change speed when they pass across the boundary between two substances with a different density, such as air and glass. This causes them to change direction, an effect called refraction.
At the boundary between two transparent substances:
- the light slows down going into a denser substance, and the ray bends towards the normal
- the light speeds up going into a less dense substance, and the ray bends away from the normal
The diagram shows how this works for light passing into, and then out of, a glass block. The same would happen for a Perspex block:
Refraction explains why an object appears to bend when it goes through water.
Refraction at the boundary between air and water