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diffraction grating.]] The visible spectrum (or sometimes '''optical spectrum''') is the portion of the ). The spectrum does not, however, contain all the colors that the human eyes and brain can distinguish. Brown and Pink are absent, for example. See Color to understand why. Wavelengths visible to the eye also pass through the " Optical Window ", the region of the electromagnetic spectrum which passes largely unattenuated through the Earth's Atmosphere (although blue light is scattered more than red light, which is the reason the sky is blue). The response of the human eye is defined by subjective testing (see CIE ), but the atmospheric windows are defined by physical measurement. The "visible window" is so called because it overlaps the human visible response spectrum; the near infrared (NIR) windows lie just out of human response window, and the Medium Wavelength IR (MWIR) and Long Wavelength or Far Infrared (FIR or LWIR) are far beyond the human response region. The eyes of many species perceive wavelengths different than the spectrum visible to the human eye. For example, many Insect s, such as Bee s, can see light in the Ultraviolet , which is useful for finding Nectar in Flower s. For this reason, plant species whose life cycles are linked to insect pollination may owe their reproductive success to their appearance in ultraviolet light. Thus, the true color of flowers may be in the ultraviolet spectrum. dispersed by a Prism into the colors of the optical spectrum.]] HISTORICAL USE OF THE TERM Two of the earliest explanations of the optical spectrum came from Newton, when he wrote his Optiks, and from Goethe, in his Theory Of Colours . Isaac Newton first used the word ''spectrum'' ( Latin for "appearance" or "apparition") in print in 1671 in describing his Experiment s in Optic s. Newton observed that, when a narrow beam of white Sunlight strikes the face of a Glass Prism at an Angle , some is 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 light. The result is that red light was bent ( Refracted ) less sharply than violet light as it passed through the prism, creating a spectrum of colors. Newton divided the spectrum into seven named colors: Red , Orange , Yellow , Green , Blue , Indigo , and Violet ; or ROYGBIV . The choice of 7 colors was numerology to accord with 7 musical notes, 7 days of the week, 7 astronomical objects. 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. Goethe contended that the continuous spectrum was a compound phenomenon. Whereas Newton narrowed the beam of light in order to isolate the phenomenon, Goethe observed that with a wider aperture, there was no spectrum - rather there were reddish-yellow edges and blue-cyan edges with white between them, and the spectrum only arose when these edges came close enough to overlap. It is now generally accepted that light is composed of Photon s (which display some of the properties of a Wave and some of the properties of a Particle ), and that all light travels at the same speed (the Speed Of Light ) 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. In some materials, known as Non-dispersive , the speed of different Frequencies (corresponding to the different colors) does not vary, and so the refractive index is a constant. However, in other (dispersive) materials, the refractive index (and thus the speed) depends on frequency in accordance with a Dispersion Relation : glass is one such material, which enables glass prisms to create an optical spectrum from white light. SPECTROSCOPY 's Atmospheric transmittance (or Opacity ) to various Wavelength s of Electromagnetic Radiation , including Visible Light .]] The scientific study of objects based on the spectrum of the light they emit is called influence on the stars analysed, as revealed by their motion paths. SEE ALSO
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