Colour

White light is a mixture of all the colours of the rainbow. Objects look coloured because they reflect some colours and absorb the rest. Filters let only their own colour through.

Key facts

  • A prism splits white light into red, orange, yellow, green, blue, indigo and violet.
  • A red object reflects red light and absorbs the others.
  • White objects reflect all colours; black ones absorb them all.
  • Mixing light: red + green = yellow, red + blue = magenta, green + blue = cyan, all three = white.

Worked examples

Example 1

What colour does a blue shirt look in red light?

  1. The shirt only reflects blue.
  2. There’s no blue in red light, so nothing is reflected: it looks black.

Answer: Black