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Historical use of the termTwo 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. Newton first used the word spectrum (Latin for "appearance" or "apparition") in print in 1671 in describing his experiments in optics. Newton observed that, when a narrow beam of 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 "corpuscles" (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. Image:Newton's colour circle.png Newton's color circle, showing the colors correlated with musical notes and symbols for the planets. Newton divided the spectrum into seven named colors: red, orange, yellow, green, blue, indigo, and violet; or ROY G. BIV. He chose seven colors out of a belief, derived from the ancient Greek philosophers, that there was a connection between the colors, the musical notes, the known objects in the solar system, and the days of the week.[1][2] 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.
It is now generally accepted that light is composed of photons (which display some of the properties of a wave and some of the properties of a particle; see Wave-particle duality), 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. SpectroscopyImage:Atmospheric electromagnetic transmittance or opacity.jpg Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light. 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. Spectral colorsAlthough the spectrum is continuous and therefore there are no clear boundaries between one color and the next, the following ranges may be used as an approximation:[3]
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References
Color vision
fr:Lumière visible ko:가시광선 no:Visuelt spektrum id:Spektrum optik it:Spettro visibile ja:可視光線 pl:Światło widzialne pt:Espectro visível ru:Видимое излучение sk:Optické spektrum fi:Värispektri sv:Spektrum zh:可见光 zh-yue:可見光 sv:Spektrum
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