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Visable spectrum

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Two of the earliest explanations of the optical spectrum came from Isaac Newton, when he wrote his Opticks, and from Goethe, in his Theory of Colours, although earlier observations had been made by Roger Bacon who first recognized the visible spectrum in a glass of water, four centuries before Newton discovered that prisms could disassemble and reassemble white light.[How to reference and link to summary or text]
Newton's colour circle

Newton's color circle, showing the colors correlated with musical notes and symbols for the planets. The spectrum aligns with the colors from violet, around the circle, to red, but Newton's circle failed to indicate the discontinuity in closing the circle from red back to violet; this is where the purple colors fall.


Newton divided the spectrum into seven named colors: red, orange, yellow, green, blue, indigo, and violet (this order being popularly memorised by schoolchildren using the mnemonic ROY G. BIV). He chose seven colors out of a belief, derived from the ancient Greek sophists, that there was a connection between the colors, the musical notes, the known objects in the solar system, and the days of the week.[2][3] The human eye is relatively insensitive to indigo's frequencies, and some otherwise well-sighted people cannot distinguish indigo from blue and violet. For this reason some commentators including Isaac Asimov have suggested that indigo should not be regarded as a color in its own right but merely as a shade of blue or violet.


All light travels at the same speed in a vacuum. The speed of light within a material is lower than the speed of light in a vacuum, and the ratio of speeds is known as the refractive index of the material. Because the refractive index (and thus the speed) of a wave in a material depends on its frequency (in accordance with a dispersion relation), light consisting of multiple frequencies—for instance white light—will be dispersed at the interface between the material and air or vacuum. Both water and glass can be used to demonstrate dispersion; a glass prism yields an optical spectrum from white light, and rainbows are an ideal example of natural refraction of the visible spectrum.

Spectral colors

Color Wavelength
violet 380–450 nm
blue 450–495 nm
green 495–570 nm
yellow 570–590 nm
orange 590–620 nm
red 620–750 nm

The familiar colors of the rainbow in the spectrum include all those colors that can be produced by visible light of a single wavelength only, the pure spectral or monochromatic colors.

Although the spectrum is continuous and therefore there are no clear boundaries between one color and the next, the ranges may be used as an approximation.[4]


Atmospheric electromagnetic transmittance or opacity


The scientific study of objects based on the spectrum of the light they emit is called spectroscopy. One particularly important application of spectroscopy is in astronomy, where spectroscopy is essential for analysing the properties of distant objects. Typically, astronomical spectroscopy utilises high-dispersion diffraction gratings to observe spectra at very high spectral resolutions. Helium was first detected through an analysis of the spectrum of the Sun; chemical elements can be detected in astronomical objects by emission lines and absorption lines; the shifting of spectral lines can be used to measure the redshift or blueshift of distant or fast-moving objects. The first exoplanets to be discovered were found by analysing the doppler shift of stars at such a high resolution that variations in their radial velocity as small as a few metres per second could be detected: the presence of planets was revealed by their gravitational influence on the stars analysed, as revealed by their motion paths.

Color display spectrum

File:Computer color spectrum.svg

Color displays (e.g., computer monitors or televisions) mix red, green, and blue color to approximate the color spectrum. In the illustration, the narrow red, green and blue bars show the relative mixture of these three colors used to produce the color directly above.

See also



  1. Cecie Starr (2005). Biology: Concepts and Applications, Thomson Brooks/Cole.
  3. Newton, Isaac (1704). Opticks.

Color vision [Edit]
Color vision | Color blindness
Monochromat | Dichromat | Trichromat | Tetrachromat | Pentachromat

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