Colour

White light is a mixture of many different colours, each with a different frequency. White light can be split up into a spectrum of these colours using a prism, a triangular block of glass or Perspex.

Light is refracted when it enters the prism, and each colour is refracted by a different amount. This means that the light leaving the prism is spread out into its different colours, a process called dispersion.

A beam of white light passes through a prism and changes into a spectrum of colours
Dispersion of white light by a prism into a spectrum

The spectrum

Here are the seven colours of the spectrum listed in order of their frequency, from the lowest frequency (fewest waves per second) to the highest frequency (most waves per second):

  • red
  • orange
  • yellow
  • green
  • blue
  • indigo
  • violet

This mnemonic is one way to remember the order: ‘Richard Of York Gave Battle In Vain’.

Coloured light

There are three primary colours in light: red, green and blue. Light in these colours can be added together to make the secondary colours magenta, cyan and yellow. All three primary colours add together make white light.

Primary colours red and blue make secondary colour magenta; blue and green make cyan; green and red make yellow, and the three colours of red, blue and green add together to make white.Primary colours of light add together to make white light, or secondary colours

The way coloured light mixes is very different from the way that paint does.

When light hits a surface, some of it is absorbed and some of it is reflected. The light that is reflected is the colour of the object in that light. For example, a blue object absorbs all the colours of the spectrum except blue: it reflects blue light.

The table gives some more examples, displaying the colour of light shining on an object, the colour(s) absorbed by an object, the colour reflected by an object in this light and the colour of an object seen in this light.

White paperRed appleGreen apple
Colours(s) that the object can reflectAllRed onlyGreen only
Appearance of object in white lightWhite (no colours absorbed)Red (all colours absorbed except red)Green (all colours absorbed except green)
Appearance of object in red lightRed (only red light to reflect)RedBlack (no green light to reflect)
Appearance of object in green lightGreen (only green light to reflect)Black (no red light to reflect)Green
Appearance of object in blue lightBlue (only blue light to reflect)Black (no red light to reflect)Black (no green light to reflect)

Objects appear black in white light because they absorb all colours and reflect none. Objects also appear black in any single colour of light if their colour is not the same as the light. For example, a green object appears black in any other light than green (or white which contains green) because there is no green light shining on it to reflect into your eyes.