Everything about The Visible Spectrum totally explained
The
visible spectrum (or sometimes called the
optical spectrum) is the portion of the
electromagnetic spectrum that's
visible to (can be detected by) the human
eye.
Electromagnetic radiation in this range of
wavelengths is called
visible light or simply
light. A typical human eye will respond to wavelengths in
air from about
380 to 750 nm. The corresponding wavelengths in
water and other media are reduced by a factor equal to the
refractive index. In terms of frequency, this corresponds to a band in the vicinity of 400–790
terahertz. A
light-adapted eye generally has its maximum sensitivity at around 555
nm (540 THz), in the
green region of the optical spectrum (see:
luminosity function). The spectrum does not, however, contain all the
colors that the human eyes and brain can distinguish. Unsaturated colors such as
pink, and
purple colors such as
magenta are absent, for example, because they can only be made by a mix of multiple wavelengths.
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 (LWIR or FIR) are far beyond the human response region.
The eyes of many
species perceive wavelengths different from the spectrum visible to the human eye. For example, many
insects, such as
bees, can see light in the
ultraviolet, which is useful for finding
nectar in
flowers. For this reason, plant species whose life cycles are linked to insect pollination may owe their reproductive success to their appearance in ultraviolet light, rather than how colorful they appear to our eyes.
History
Two 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, although earlier observations had been made by
Roger Bacon who first recognized the visible spectrum in a glass of water, four centuries before Newton discovered that prisms could disassemble and reassemble white light.
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.
Newton divided the spectrum into seven named colors:
red,
orange,
yellow,
green,
blue,
indigo, and
violet (this order being popularly memorised by schoolchildren using the mnemonic
ROY G. BIV). He chose seven colors out of a belief, derived from the ancient Greek
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. The human eye is relatively insensitive to indigo's frequencies, and some otherwise well-sighted people can't distinguish indigo from blue and violet. For this reason some commentators including
Isaac Asimov have suggested that indigo shouldn't be regarded as a color in its own right but merely as a shade of blue or violet.
Johann Wolfgang von 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.
All light travels at the same speed 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. Because the
refractive index (and thus the speed) of a wave in a material depends on its
frequency (in accordance with a
dispersion relation), light consisting of multiple frequencies—for instance white light—will be
dispersed at the interface between the material and air or vacuum. Both water and glass can be used to demonstrate dispersion; a glass
prism yields an optical spectrum from white light, and
rainbows are an ideal example of natural refraction of the visible spectrum.
Spectral colors
The familiar colors of the
rainbow in the spectrum include all those colors that can be produced by visible light of a single wavelength only, the
pure spectral or
monochromatic colors.
Although the spectrum is continuous and therefore there are no clear boundaries between one color and the next, the ranges may be used as an approximation.
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.
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.
Color display spectrum
Color displays (for example,
computer monitors or
televisions) mix
red,
green, and
blue color to approximate the color spectrum. In the illustration, the narrow red, green and blue bars show the relative mixture of these three colors used to produce the color directly above.
Further Information
Get more info on 'Visible Spectrum'.
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