Color

Physics of color

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The colors of the visible light spectrum.

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color

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wavelength interval

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frequency interval

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red

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~ 625-740 nm

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~ 480-405 THz

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orange

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~ 590-625 nm

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~ 510-480 THz

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yellow

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~ 565-590 nm

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~ 530-510 THz

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green

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~ 500-565 nm

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~ 600-530 THz

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cyan

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~ 485-500 nm

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~ 620-600 THz

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blue

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~ 440-485 nm

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~ 680-620 THz

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violet

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~ 380-440 nm

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~ 790-680 THz

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Continuous optical spectrum

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Designed for monitors with gamma 1.5.

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Computer "spectrum"

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The bars below show the relative intensities of the threecolors mixed to make the color immediately above.

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Color, frequency, and energy of light.

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Color

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lambda ,!/nm

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u ,!/1014 Hz

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u_b ,!/104 cm-1

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E ,!/eV

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E ,!/kJ mol-1

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Infrared

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>1000

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Red

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700

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4.28

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1.43

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1.77

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171

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Orange

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620

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4.84

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1.61

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2.00

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193

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Yellow

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580

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5.17

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1.72

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2.14

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206

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Green

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530

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5.66

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1.89

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2.34

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226

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Blue

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470

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6.38

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2.13

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2.64

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254

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Violet

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420

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7.14

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2.38

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2.95

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285

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Near ultraviolet

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300

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10.0

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3.33

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4.15

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400

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Far ultraviolet

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>15.0

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>5.00

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>6.20

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>598

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Electromagnetic radiation is a mixture of radiation of different wavelengths and intensities. When this radiation has a wavelength inside the human visibility range (approximately from 380 nm to 740 nm), it is known as light within the (human) visible spectrum. The light's spectrum records each wavelength's intensity. The full spectrum of the incoming radiation from an object determines the visual appearance of that object, including its perceived color. As we will see, there are many more spectra than color sensations; in fact one may formally define a color to be the whole class of spectra which give rise to the same color sensation, although any such definition would vary widely among different species and also somewhat among individuals intraspecifically.

Related Topics:
Electromagnetic radiation - Wavelength - Nm - Visible spectrum

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A surface that diffusely reflects all wavelengths equally is perceived as white, while a dull black surface absorbs all wavelengths and does not reflect (for mirror reflection this is different: a proper mirror also reflects all wavelengths equally, but is not perceived as white, while shiny black objects do reflect).

Related Topics:
Reflects - White - Black - Mirror

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The familiar colors of the rainbow in the spectrum—named from the Latin word for appearance or apparition by Isaac Newton in 1671—contains all those colors that consist of visible light of a single wavelength only, the pure spectral or monochromatic colors.

Related Topics:
Rainbow - Spectrum - Latin - Isaac Newton - 1671

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The frequencies are approximations and given in terahertz (THz). The wavelengths, valid in vacuum, are given in nanometers (nm). A list of other objects of similar size is available.

Related Topics:
Terahertz - Vacuum - Nanometers - Other objects of similar size

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Important note

The color table should not be interpreted as a definite list – the pure spectral colors form a continuous spectrum, and how it is divided into distinct colors is a matter of taste and culture.

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Similarly, the intensity of a spectral color may alter its perception considerably; for example, a low-intensity orange-yellow is brown, and a low-intensity yellow-green is olive-green.

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Spectral versus non-spectral colors

Most light sources are not pure spectral sources; rather they are created from mixtures of various wavelengths and intensities of light. To the human eye, however, there is a wide class of mixed-spectrum light that is perceived the same as a pure spectral color. In the table above, for instance, when your computer screen is displaying the "orange" patch, it is not emitting pure light at a fixed wavelength of around 600 nm (which is in fact not a thing most computer screens are even able to do). Rather, it is emitting a mixture of about two parts red to one part green light. Were you to print this page on a color printer, the orange patch on the paper, when lit with white light, would reflect yet another, more continuous spectrum. We cannot see those differences (although many animals can), and the reason has to do with the pigments that make up our color vision cells (see below).

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A useful quantification of this property is the dominant wavelength, which matches a wavelength of spectral light to a non-spectral source that evokes the same color perception. Dominant wavelength is the formal background for the popular concept of hue.

Related Topics:
Dominant wavelength - Hue

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In addition to the many light sources that can appear to be pure spectral colors but are actually mixtures, there are many color perceptions that by definition cannot be pure spectral colors due to desaturation or because they are purples (which are a mixture of red and violet light, from either end of the spectrum). Some examples of necessarily non-spectral colors are the achromatic colors (black, gray and white) and other colors such as pink, tan and magenta.

Related Topics:
Pink - Magenta

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See metamerism (color) for a basic introduction as to why color matching challenges exist.

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Color in the wave equation

The wave equation describes the behaviour of light and so we should be able to describe color spectra in terms of the mathematical properties of the solutions of the wave equation. However, to understand which particular color perception will arise from a particular physical spectrum requires knowledge of the specific retinal physiology of the observer. For completeness, we include a simple equation for light traveling in a vacuum:

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Utt=c2(Uxx+Uyy+Uzz)

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where the subscripts denote partial derivatives and c is the speed of light. If we fix (x,y,z) a point in space and look at the solution u(x,y,z,t) as a function of t, we obtain a signal. If we take the Fourier transform of this signal, we obtain a frequency decomposition as described above. Each frequency has an amplitude and phase. The frequency multiplied by Planck's constant h determines the energy of a photon of the relevant component. The square of the amplitude represents the intensity, which is the amount of energy transmitted per second, through a unit area of a surface perpendicular to the light propagation. The phase information is much more mysterious because it is difficult to measure and observe. Humans cannot perceive phase effects of light except in special cases of interference (e.g. see thin-film optics) where phase effects lead to perceptible amplitude changes. Most light has randomly distributed phases, but lasers are more efficient when the photons all have the same phase.

Related Topics:
Partial derivative - Signal - Fourier transform - Planck's constant - Interference - Thin-film optics - Lasers

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