Key facts

  • When light travels from a less dense to a more dense material, it gets slower and bends towards the normal. e.g. air to glass.
  • When light travels from a more dense to a less dense material, it gets faster and bends away from the normal. e.g. glass to air.
  • The greater the change of speed, the more the light is bent or refracted.

One way of remembering this is to use the word FAST.

If light gets Faster it bends Away from the normal.

If light gets Slower it bends Towards the normal.

LetterMeaning
FFaster
AAway
SSlower
TTowards

If light is incident along the normal when it passes from air into glass it still slows down but its direction does not change – it passes straight through.

Likewise, if light is incident along the normal when it passes from glass into air it still speeds up but its direction does not change – it passes straight through.

A ray of light incident along the normal passes straight through without being refracted

A ray of light incident along the normal passes straight through without being refracted.