The visible 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. A typical human eye will respond to wavelengths from about 390
to 700 nm. In terms of frequency, this corresponds to a band in the vicinity of 430–790 THz.
The spectrum does not, however, contain all the colors that the human eyes and brain can
distinguish. Unsaturated colors such as pink, or purple variations such as magenta, are
absent, for example, because they can be made only by a mix of multiple wavelengths. Colors
containing only one wavelength are also called pure colors or spectral colors.
Visible wavelengths pass through the "optical window", the region of the electromagnetic
spectrum that allows wavelengths to pass largely unattenuated through the Earth's atmosphere.
An example of this phenomenon is that clean air scatters blue light more than red wavelengths,
and so the midday sky appears blue. The optical window is also called the visible window because
it overlaps the human visible response spectrum. The near infrared window lies just out of
the human vision, as well as the Medium Wavelength IR window and the Long Wavelength or Far Infrared
window though other animals may experience them.
History
In the 13th century, Roger Bacon theorized that rainbows were produced by a similar process
to the passage of light through glass or crystal. In the 17th century, Isaac Newton discovered
that prisms could disassemble and reassemble white light, and described the phenomenon
in his book Opticks. He was the first to use the word spectrum in this sense 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 light to be made up of "corpuscles" of different colors, with the different colors
of light moving at different speeds in transparent matter, red light moving more quickly than
violet in glass. The result is that red light bends less sharply than violet as it passes
through the prism, creating a spectrum of colors.
Newton divided the spectrum into seven named colors: red, orange, yellow, green, blue,
indigo, and violet. He chose seven colors out of a belief, derived from the ancient
Greek sophists, of there being 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 cannot distinguish
indigo from blue and violet. For this reason, some later 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. However, the evidence indicates that what Newton meant by "indigo"
and "blue" does not correspond to the modern meanings of those color words. Comparing Newton's
observation of prismatic colors to a color image of the visible light spectrum shows
that "indigo" corresponds to what is today called blue, whereas "blue" corresponds to
cyan. In the 18th century, Goethe wrote about optical
spectra in his Theory of Colours. Goethe used the word spectrum to designate a ghostly optical
afterimage, as did Schopenhauer in On Vision and Colors. Goethe argued that the continuous
spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate
the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather
reddish-yellow and blue-cyan edges with white between them. The spectrum appears only when
these edges are close enough to overlap. In the early 19th century, the concept of
the visible spectrum became more definite, as light outside the visible range was discovered
and characterized by William Herschel and Johann Wilhelm Ritter, Thomas Young, Thomas
Johann Seebeck, and others. Young was the first to measure the wavelengths of different
colors of light, in 1802. The connection between the visible spectrum
and color vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th
century. Their theory of color vision correctly proposed that the eye uses three distinct
receptors to perceive color.
Animal color vision Many species can see light with frequencies
outside the human "visible spectrum". Bees and many other insects can detect ultraviolet
light, which helps them find nectar in flowers. Plant species that depend on insect pollination
may owe reproductive success to their appearance in ultraviolet light rather than how colorful
they appear to humans. Birds, too, can see into the ultraviolet, and some have sex-dependent
markings on their plumage that are visible only in the ultraviolet range. Many animals
that can see into the ultraviolet range, however, cannot see red light or any other reddish
wavelengths. Bees' visible spectrum ends at about 590 nm, just before the orange wavelengths
start. Birds, however, can see some red wavelengths, although not as far into the light spectrum
as humans. It is an incorrect popular belief that the common goldfish is the only animal
that can see both infrared and ultraviolet light, but goldfish cannot see infrared light.
Spectral colors Colors that can be produced by visible light
of a narrow band of wavelengths are called pure spectral colors. The various color ranges
indicated in the diagram on the right are an approximation: The spectrum is continuous,
with no clear boundaries between one color and the next.
Spectroscopy
Spectroscopy is the study of objects based on the spectrum of color they emit, absorb
or reflect. Spectroscopy is an important investigative tool in astronomy, where scientists use it
to analyze the properties of distant objects. Typically, astronomical spectroscopy uses
high-dispersion diffraction gratings to observe spectra at very high spectral resolutions.
Helium was first detected by 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 Doppler shift of distant objects.
Color display spectrum
Color displays cannot reproduce all colors discernible by a human eye. Colors outside
the color gamut of the device, such as most spectral colors, can only be approximated.
For color-accurate reproduction, a spectrum can be projected onto a uniform gray field.
The resulting mixed colors can have all their R,G,B coordinates non-negative, and so can
be reproduced without distortion. This accurately simulates looking at a spectrum on a gray
background. See also
High-energy visible light
References