<gpm119@shaw.ca> wrote in message

When light travels from air into glass, it slows down.

As a result, if light hits a glass surface at any angle other than
straight-on, the light changes direction.

A similar change occurs again when light exits the glass and re-enters the
air.

If the two (flat) surfaces are exactly parallel, the net change in direction
is zero.

If the two surfaces are not parallel, the light bends toward the thicker
edge.

So a clear piece of glass or plastic with two surfaces at an angle makes a
prism.

-MT