Number of Colours in Spectrum or Rainbow
A spectrum obtained using a glass prism and a point source is a continuum of wavelengths without bands. The number of colours that the human eye is able to distinguish in a spectrum is in the order of 100. Accordingly, the Munsell colour system (a 20th century system for numerically describing colours, based on equal steps for human visual perception) distinguishes 100 hues. The apparent discreteness of primary colours is an artefact of human perception and the exact number of primary colours is a somewhat arbitrary choice. The human brain tends to divide them into a small number—often seven—of primary colours.
| Red | Orange | Yellow | Green | Blue | Indigo | Violet |
| The seven primary colours | ||||||
Newton originally (1672) divided the spectrum into five primary colours: red, yellow, green, blue and violet. Later he included orange and indigo, giving seven primary colours by analogy to the number of notes in a musical scale. Note that what Newton called blue and indigo are what we call cyan and blue today respectively.
The colour pattern of a rainbow is different from a spectrum, and the colours are less saturated. There is spectral smearing in a rainbow owing to the fact that for any particular wavelength, there is a distribution of exit angles, rather than a single unvarying angle. In addition, a rainbow is a blurred version of the bow obtained from a point source, because the disk diameter of the sun (0.5°) cannot be neglected compared to the width of a rainbow (2°). The number of colour bands of a rainbow may therefore be different from the number of bands in a spectrum, especially if the droplets are either large or small. Therefore, the number of colours of a rainbow is variable. If, however, the word rainbow is used inaccurately to mean spectrum, it is the number of primary colours in the spectrum.
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