Visible spectrum

White light dispersed by a prism into the colors of the optical spectrum.

The visible spectrum is the portion of the electromagnetic spectrum that is 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 from about 390 to 750 nm.^[1] In terms of frequency, this corresponds to a band in the vicinity of 400-790 THz. 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, or purple variations such as magenta, are absent, for example, because they can only be made by a mix of multiple wavelengths.

Visible wavelengths also pass through the "optical window", the region of the electromagnetic spectrum that passes largely unattenuated through the Earth's atmosphere. Clean air scatters blue light more than wavelengths toward the red, which is why the mid-day sky appears blue. The human eye's response is defined by subjective testing, but 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.

Many species can see frequencies which fall outside the "visible spectrum". Bees and many other insects can see light in the ultraviolet, 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 us. Birds too can see into the ultraviolet (300–400 nm), and some have sex-dependent markings on their plumage, which are only visible in the ultraviolet range.^[2][3]

History

Newton's color circle, from Opticks of 1704, showing the colors correlated with musical notes. The spectral colors from red to violet are divided by the notes of the musical scale, starting at D. The circle completes a full octave, from D to D. Newton's circle places red, at one end of the spectrum, next to violet, at the other. This reflects the fact that non-spectral purple colors are observed when red and violet light are mixed.

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.^[4]

Newton first used the word spectrum (Latin for "appearance" or "apparition") in print in 1671 in describing his experiments in optics. The word "spectrum" [Spektrum] was strictly used to designate a ghostly optical afterimage by Goethe in his Theory of Colors and Schopenhauer in On Vision and Colors. 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. The result is that red light bends (refracted) 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. (Some schoolchildren memorize this order using the mnemonics ROY G. BIV or Richard Of York Gave Battle In Vain.) 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.^[5][6] 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 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 only appears 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—ultraviolet and infrared—was discovered and characterized by William Herschel, Johann Wilhelm Ritter, Thomas Young, Thomas Johann Seebeck, and others.^[7] Young was the first to measure the wavelengths of different colors of light, in 1802.^[8]

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.

Spectral colors

Color	Frequency	Wavelength
violet	668–789 THz	380–450 nm
blue	631–668 THz	450–475 nm
cyan	606–630 THz	476–495 nm
green	526–606 THz	495–570 nm
yellow	508–526 THz	570–590 nm
orange	484–508 THz	590–620 nm
red	400–484 THz	620–750 nm

Colors that can be produced by visible light of a single frequency or wavelength (monochromatic light) are referred to as the pure spectral colors.

Although the spectrum is continuous, with no clear boundaries between one color and the next, the ranges may be used as an approximation.^[9]

Spectroscopy

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light.

Spectroscopy is the study of objects based on the spectrum of color they emit or absorb. 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 analyzing 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 red shift or blue shift of distant or fast-moving objects. The first exoplanets were discovered by analyzing the Doppler shift of stars at a resolution that revealed variations in radial velocity as small as a few meters per second. The presence of planets was revealed by their gravitational influence on the motion of the stars.

Color display spectrum

Color spectrum of the complete RGB color wheel generated in a display device.

Color displays (e.g., computer monitors and televisions) mix red, green, and blue color to create colors within their respective color triangles, and so can only approximately represent spectral colors, which are in general outside any color triangle.

A render of the visible spectrum into the sRGB color space on a gray background.

Colors outside the color gamut of the display device result in negative values. If color accurate reproduction of the spectrum is desired, negative values can be avoided by rendering the spectra on a gray background. This gives an accurate simulation of looking at a spectrum on a gray background.^[10]

See also

• Color vision
• High-energy visible light
• Deep Color

References