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The optical spectrum (light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. There are no exact bounds to the optical spectrum; a typical human eye will respond to wavelengths from 400 to 700 nm, although some people may be able to perceive wavelengths from 380 to 780 nm. A light-adapted eye typically has its maximum sensitivity at ~555 nm, in the yellow region of the optical spectrum.
Wavelengths visible to the eye are defined by the spectral range of 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). Electromagnetic radiation outside the optical wavelength range is almost entirely absorbed by the atmosphere.
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Historical use of the term
Sir Isaac Newton first used the Latin word spectrum (appearance or apparition) in print in 1671. He was describing the phenomenon of colored bands dispersing from white sunlight passing through a prism.
Explanation of Newton's experiment
When a beam of sunlight strikes the face of a glass prism at an angle, some is reflected and some of the beam passes into the glass. All light travels at the same speed in a vacuum, but in transparent matter different colors (frequencies) move at different speeds. Red light moves more quickly in glass than violet light and it bends (refracts) less sharply. A triangular prism is shaped to bend the light twice, and disperse it as much as possible. The result is the spectrum of colors.
Spectroscopy
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. The first exoplanets to be discovered were found by analysing the doppler shift of stars at such 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 motion of the stars analysed.
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