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== History ==
[[Image:Newton's colour circle.png|thumb|250px|right|Newton's color circle, from ''Opticks'' of 1704, showing the colors correlated with [[musical note]]s. The spectral colors from red to violet are divided by the notes of the musical scale, starting at D. The circle completes a full [[octave]], from D to D. Newton's circle places red, at one end of the spectrum, next to violet, at the other. This reflects the fact that non-spectral [[purple]] colors are observed when red and violet light are mixed.]]
An earliest reference to sunlight composed of seven components can be found in Surya Ashtaka hymns of [[Samba Purana]] which is a Vedic text in ancient India. Here the sun is identified as a lustrous chariot riding seven horses, white in color and highly radiant like a hibiscus flower<ref>{{cite book|url=http://www.celextel.org/stotras/grahas/suryaashtakam.html}}</ref> . The text belongs to [[Śruti]] tradition when the teachings were oral, hence true dating is unknown. The written portion can be dated to belong to the [[Vedic period]] which is around 1500 BC.
In 17th century the explanations of the optical spectrum came from [[Isaac Newton]], when he wrote his book ''[[Opticks]]''. In 18th century [[Johann Wolfgang Goethe|Goethe]] wrote about optical spectra in his ''[[Theory of Colours (book)|Theory of Colours]]''. Earlier observations had been made by [[Roger Bacon]] who recognized the visible spectrum in a glass of water, four centuries before Newton discovered that prisms could disassemble and reassemble white light.<ref>{{cite book|url=http://books.google.com/?id=j8BCAAAAIAAJ&pg=PA185&dq=%22roger+bacon%22+prism|title=The Science of Logic: An Inquiry Into the Principles of Accurate Thought |first=Peter|last=Coffey|year=1912|publisher=Longmans}}</ref>
Newton first used the word ''spectrum'' ([[Latin]] for "appearance" or "apparition") in print in 1671 in describing his [[experiment]]s in [[optics]]. The word "spectrum" [Spektrum] was strictly used to designate a ghostly optical [[afterimage]] by [[Johann Wolfgang von Goethe|Goethe]] in his ''[[Theory of Colours (book)|Theory of Colors]]'' and [[Arthur Schopenhauer|Schopenhauer]] in ''[[On Vision and Colors]]''. Newton observed that when a narrow beam of [[sunlight]] strikes the face of a glass [[Dispersive prism|prism]] at an angle, some is [[Reflection (physics)|reflected]] and some of the beam passes into and through the glass, emerging as different colored bands. Newton hypothesized that light was made up of "[[corpuscle]]s" (particles) of different colors, and that the different colors of light moved at different speeds in transparent matter, with red light moving more quickly in glass than violet. The result is that red light bends ([[refraction|refracted]]) less sharply than violet as it passes through the prism, creating a spectrum of colors.
Newton divided the spectrum into seven named colors: [[red]], [[Orange (color)|orange]], [[yellow]], [[green]], [[blue]], [[indigo]], and [[Violet (color)|violet]]. (Often abbreviated [[ROY G. BIV]]) He chose seven colors out of a belief, derived from the [[Ancient Greece|ancient Greek]] [[Sophism|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.<ref>{{cite web|url=http://home.vicnet.net.au/~colmusic/opticks3.htm |title=Music For Measure: On the 300th Anniversary of Newton's ''Opticks'' |accessdate=2006-08-11 |last=Hutchison |first=Niels |year=2004 |work=Colour Music }}</ref><ref>{{cite book |last=Newton |first=Isaac |authorlink=Isaac Newton |title=[[Opticks]] |year=1704 }}</ref> 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.
[[Johann Wolfgang von Goethe]] argued that the continuous spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum, but rather reddish-yellow and blue-cyan edges with [[white]] between them. The spectrum only appears when these edges are close enough to overlap.
In the early 19th century, the concept of the visible spectrum became more definite, as light outside the visible range was discovered and characterized by [[William Herschel]] ([[infrared]]) and [[Johann Wilhelm Ritter]] ([[ultraviolet]]), [[Thomas Young (scientist)|Thomas Young]], [[Thomas Johann Seebeck]], and others.<ref>{{cite book
| title = The Cambridge History of Science: The Modern Physical and Mathematical Sciences
| volume = 5
| author = Mary Jo Nye (editor)
| publisher = Cambridge University Press
| year = 2003
| isbn = 9780521571999
| page = 278
| url = http://books.google.com/?id=B3WvWhJTTX8C&pg=PA278&dq=spectrum+%22thomas+young%22+herschel+ritter
}}</ref>
Young was the first to measure the wavelengths of different colors of light, in 1802.<ref>{{cite book
| title = Lines of light: the sources of dispersive spectroscopy, 1800-1930
| author = John C. D. Brand
| publisher = CRC Press
| year = 1995
| isbn = 9782884491631
| page = 30–32
| url = http://books.google.com/?id=sKx0IBC22p4C&pg=PA30&dq=light+wavelength+color++young+fresnel
}}</ref>
The connection between the visible spectrum and [[color vision]] was explored by Thomas Young and [[Hermann von Helmholtz]] in the early 19th century. Their [[Young–Helmholtz theory|theory of color vision]] correctly proposed that the eye uses three distinct receptors to perceive color.
==Spectral colors==
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