Visible spectrum
[[Image:spectrum4websiteEval.png|centre|approximation to the white light spectrum dispersed via an EdmundScientific Spectroscope or a 4x8 sheet of diffraction grating.]]
The
visible spectrum (or sometimes
optical spectrum) is the portion of the
electromagnetic spectrum that is
visible to the human
eye. Electromagnetic radiation in this range of wavelengths is called
visible light or simply
light. There are no exact bounds to the visible 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 around 555
nm, 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.
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 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. Thus, the true color of flowers may be in the ultraviolet spectrum.
 |
White light dispersed by a prism into the colors of the optical spectrum. |
Two of the earliest explanations of the optical spectrum came from Newton, when he wrote his
Opticks, 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
experiments in
optics. 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 "
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; or
ROYGBIV. 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.
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.
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.
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 vision*
Frequency*
High energy visible light*
Rainbow*
ROYGBIV*
Rydberg formula*
Wavelength*
Theory of Colours
