Bands of colour

An observer sees bands of colour when a continuous range of wavelengths within the visible spectrum appear to be a single colour.

In the presence of a rainbow, an observer will typically see six bands of colour (red, orange, yellow, green, blue and violet) rather than a different colour corresponding with each wavelength.

  • The fact that we see distinct bands of colour is often described as an artefact of human colour vision.
  • There is no property belonging to the visible part of the electromagnetic spectrum that causes bands of colour to appear to an observer.
  • The visible spectrum is composed of a continuous range of wavelengths between red and violet that produce a continuous range of corresponding colours.
  • In experimental situations, human observers can distinguish between spectral colours corresponding with many thousands of different wavelengths of light.
About colour and visual perception
  • Colour is not a property of electromagnetic radiation, but a feature of visual perception.
  • The human eye, and so human perception, is tuned to the range of wavelengths of light that make up the visible spectrum and so to the corresponding spectral colours between red and violet.
  • Light, however, is rarely of a single wavelength, so when an observer notices a red ball they are probably seeing a range of similar wavelengths of light within the visual spectrum.
  • Perception of colour is a subjective process as our eyes respond to stimuli produced by incoming light but each of us responds differently.
About rainbows and bands of colour
  • There seem to be a number of reasons that an observer looking at phenomena such as rainbows  sees bands of colour:
    • The human perceptual system prioritise the simplification of colour information rather than observing a gentle gradient across the spectrum.
    • Our eyes respond to colours in terms of their relative brightness as well as their hue when presented with a section or the whole of the visible spectrum.
    • Observers look for colours they know and recognise.
    • Cone cells in our eyes are particularly sensitive to red, green and blue wavelengths because of the trichromatic nature (trichromacy) of human vision.
About bands of colour, spectral and non-spectral colours
    Bands of colour
    • Bands of colour are composed of a continuous range of wavelengths, so for example:
      • A continuous range of wavelengths between 750 – 620 nanometres (nm) typically appear red to an observer.
      • Wavelengths between 590 – 570 nm will typically appear yellow.
      • A continuous range of wavelengths between 450 – 380 nm will typically appear violet.
    Spectral colours
    • A spectral colour is a colour that is evoked by a single wavelength of light (or narrow band of wavelengths) within the visible spectrum.
    • Spectral colours are the colours red to violet.
    • Diagrams of the spectral colour model are linear and may show colours selected:
      • Using equal and incremental steps in wavelength.
      • According to equal and incremental steps in the appearance of colours.
    Non-spectral colours
    • Non-spectral colours are produced by additive mixtures of wavelengths of light.
    • Examples of non-spectral colours produced by two spectral colours are:
      • Purple – produced by mixing wavelengths corresponding with red and violet. Red (740nm) and violet (400nm) are at the extreme limits of the visible spectrum.
      • Magenta –  produced by mixing red (660nm) and blue (490nm).
      • Mauve – produced by mixing orange (600nm) and blue (450nm).
      • Examples of non-spectral colours produced by three spectral colours are:
        • Tints
        • Greys
        • Shades
        • So all achromatic colours are non-spectral colours.
    • Whilst both spectral and non-spectral colours are produced by mixing a combination of colours corresponding with different wavelengths of light:
      • The RGB colour model produces a full gamut of colours by mixing red, green and blue primary colours in different proportions.
      • The CMY colour model produces a full gamut of colours by mixing cyan, magenta and yellow primary colours in different proportions.

    An observer perceives bands of colour when visible light separates into its component wavelengths and the human eye distinguishes between some colours better than others.

        • The human eye and brain together translate light into colour.
        • When sunlight is dispersed by rain and forms a rainbow, an observer often distinguishes red, orange, yellow, green, blue and violet bands of colour.
        • Although a rainbow contains electromagnetic waves with all possible wavelengths between red and violet, some ranges of wavelengths appear more intense to a human observer than others.