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.
Read more about this topic: Rainbow
Famous quotes containing the words number of, number, colours and/or rainbow:
“Again, the great number of cultivated men keep each other up to a high standard. The habit of meeting well-read and knowing men teaches the art of omission and selection.”
—Ralph Waldo Emerson (1803–1882)
“... in every State there are more women who can read and write than the whole number of illiterate male voters; more white women who can read and write than all Negro voters; more American women who can read and write than all foreign voters.”
—National Woman Suffrage Association. As quoted in History of Woman Suffrage, vol. 4, ch. 13, by Susan B. Anthony and Ida Husted Harper (1902)
“Your wits can’t thicken in that soft moist air, on those white springy roads, in those misty rushes and brown bogs, on those hillsides of granite rocks and magenta heather. You’ve no such colours in the sky, no such lure in the distances, no such sadness in the evenings. Oh the dreaming! the dreaming! the torturing, heart-scalding, never satisfying dreaming, dreaming, dreaming, dreaming!”
—George Bernard Shaw (1856–1950)
“The true harvest of my daily life is somewhat as intangible and indescribable as the tints of morning or evening. It is a little star-dust caught, a segment of the rainbow which I have clutched.”
—Henry David Thoreau (1817–1862)