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 unique colour corresponding with each wavelength.

  • The phenomenon of perceiving distinct colour bands is typically attributed to the characteristics of human colour vision, or as an artefact of human colour vision.
  • There is no property belonging to the visible part of the electromagnetic spectrum that that results in the appearance of bands of colour 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 hundreds of different wavelengths of light.
About colour & visual perception
  • Colour is not a property of electromagnetic radiation, but rather a characteristic of visual perception.
  • The human eye, and therefore human perception, is sensitive to the range of light wavelengths that constitute the visible spectrum, including the corresponding spectral colours from red to 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 are several reasons why an observer looking at phenomena like rainbows perceives bands of colour.
    • The human perceptual system tends to simplify colour information rather than perceiving a smooth gradient across the spectrum.
    • Our eyes respond to colours based on their relative brightness and hue when presented with a portion or the entirety of the visible spectrum.
    • Observers tend to search for colours they are familiar with and can recognize and name.
    • Cone cells in our eyes are especially sensitive to red, green, and blue wavelengths due to 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.