Things appear coloured to an observer because colour corresponds with a property of light that is visible to the human eye. The visual experience of colour is associated with words such as red, blue, yellow, etc.

Why we see colour
Light and colour
  • Light is electromagnetic radiation (radiant energy), which, detached from its source, is transported by electromagnetic waves (or their quanta, photons) and propagates through space. Even if humans had never evolved, electromagnetic radiation would have been emitted by stars since the formation of the first galaxies over 13 billion years ago.
  • The experience of colour is a feature of human vision that depends first of all on the construction of our eyes and the wavelength, frequency and brightness of visible light that strikes the retina at the back of each eye.
  • Because colour is a visual experience that is specific to each and every one of us at any given moment, we share our experiences of colour using language but to make sure we are talking about the same colour we use examples.
White light
  • The name given to light that contains all wavelengths of the visible spectrum at equal intensity is white light.
  • The term white light doesn’t mean light is white as it travels through the air. As light travels through the air it is invisible to our eyes.
  • When white light strikes a neutral coloured object, and all wavelengths are reflected, then it appears white to an observer.
Observation of colour
  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet.
  • It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of colour.
  • The colour an observer sees depends on:
    • The wavelengths of visible light emitted by a light source.
    • The wavelengths transmitted through a medium or reflected off an object.
  • Although a human observer can distinguish between many thousands of wavelengths of light in the visible spectrum our brains often produce the impression of bands of colour.
  • As light travels from one medium to another, such as from air to glass, the wavelength changes but the frequency remains the same so the colour seen by an observer remains the same.