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:

3/3: wave enters block at angle to surface. Angle to normal labelled 'angle of incidence' when entering & 'angle of refraction' as bent inside block. Angle of refraction less than angle of incidence.Refraction in a glass block. When light passes from air through a block with parallel sides, it emerges parallel to the path of the light ray that entered it.

Refraction explains why an object appears to bend when it goes through water.

Pencil appearing bent due to light refraction of water
Refraction at the boundary between air and water