The optical spectrum (light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the humaneye. 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.
Historical use of the term
Sir Isaac Newton first used the word spectrum in 1666 to refer to the celebrated Phenomenon of Colours in which he demonstrated that "white light" was actually made up of a spectrum of colors. He refracted "white light" by projecting a slit of sunlight into a glass prism.
Prism showing the spectrum of colors which make up "white light":.
You can simulate his discovery with a slit or spot of "white light" projected onto a triangular prism. It will refract the differing wavelengths at different angles/speeds, resulting in a projected spectrum of the light's constituent colors. This is because the glass of which the prism is made is a dispersive medium. It's triangular shape allows the longer (red) wavelengths to pass through first, then the green, then the blue. Blue is the higher frequency color, so it stays in the prism longer, and bends and exits the prism at the steepest angle than the red or green in the prism. The resulting image is a saturated spectrum or rainbow of colored lights projected on the wall. However, with the advent of diffraction gratings the easiest is to purchase a $12 spectroscope at a kids science store.
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.
As shown on the right, the component colors of the visible portion can be separated by passing sunlight through a prism, which acts to bend the light in differing degrees according to wavelength.
Electromagnetic radiation such as visible light is commonly treated as a wave phenomenon, characterized by a wavelength or frequency.
The visible region of the spectrum comprises photon energies of 36 to 72 kcal/mole, and the near ultraviolet region, out to 200 nm, extends this energy range to 143 kcal/mole.
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