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White

White

White is a color (more accurately it contains all the colors of the visible spectrum and is sometimes described as an achromatic color—black is the absence of color) that has high brightness but zero hue. The impression of white light can be created by mixing (via a process called "additive mixing") appropriate intensities of the primary color spectrum: red, green and blue, but it must be noted that the illumination provided by this technique has significant differences from that produced by incandescence (see below).

Color

Paint

In painting, white can be created by reflecting ambient light from a white pigment. White when mixed with black produces gray. To art students, the use of white can present particular problems, and there is at least one training course specialising in the use of white in art. There are various white pigments. Lead white, also known as flake white, is the traditional white pigment, but it is not much used now as it is toxic. Non-toxic alternatives are zinc white and titanium white. They are made from zinc oxide and titanium dioxide respectively.

White light

Until Newton's work became accepted, most scientists believed that white was the fundamental color of light; and that other colors were formed only by adding something to light. Newton demonstrated that white was formed by combining the other colors. In the science of lighting, there is a continuum of colors of light that can be called "white". One set of colors that deserve this description are the colors emitted, via the process called incandescence, by a black body at various relatively-high temperatures. For example, the color of a black body at a temperature of 2848 kelvins matches that produced by domestic incandescent light bulbs. It is said that "the color temperature of such a light bulb is 2848 K". The white light used in theatre illumination has a color temperature of about 3200 K. Daylight has a nominal color temperature of 5400 K (called equal energy white), but can vary from a cool red up to a bluish 25,000 K. Not all black body radiation can be considered white light: the background radiation of the universe, to name an extreme example, is only a few kelvins and is quite invisible.

Standard whites

Standard whites are often defined with reference to the International Commission on Illumination's (CIE's) chromaticity diagram. These are the D series of standard illuminants. Illuminant D65, originally corresponding to a color temperature of 6,500 K, is taken to represent standard daylight.

Computers

Computer displays often have a color temperature control, allowing the user to select the color temperature (usually from a small set of fixed values) of the light emitted when the computer produces the electrical signal corresponding to "white". The RGB coordinates of white are 255 255 255.

Usage, symbolism, colloquial expressions

In general, since white is opposite of black, it is often used with positive connotation. Many negative expressions with "black" have an equivalent positive expression with "white". For example, whitehat describes a person who is ethically opposed to the abuse of computer systems, in contrast with blackhat. White has also many other meanings:
- The term white is often used in the West to denote "race" for so-called Caucasian people, i.e. people of European/West Asian descent with light skin color, whose skin color actually ranges from pink to pale brown, and overlaps with some people that might be classified as "Blacks". For more details, see Whites.
- White noise, in acoustics, is a sibilant sound that is often a nuisance, although it can also be deliberately created for test purposes.
- Whitewash, figuratively, means an attempt to obscure the truth by issuing a blanket of lies. See propaganda.
- Whiteout is a weather condition in which visibility is reduced and surface definition lost in snowy environments.
- In Chinese and Indian tradition, white is the color of mourning, death, and ghosts.
- In English heraldry, white or silver (color) signified brightness, purity, virtue, and innocence. (The American Girls Handy Book, p.369)
- White is the traditional color of bridal dress in both western (European) and Japanese weddings. In Western weddings, a white dress is symbolic of purity (the bride has not engaged in pre-marital sex)
- White is often associated with Conservatism (as opposed to Communism), particularly in the years following World War I, with civil wars fought between "Reds" and "Whites", for instance the Civil War in Russia and the Civil War in Finland.
- A white flag is an international sign of either surrender, or truce, that is, it is a sign of peaceful intent, typically at time of war.
- A white paper can be an authoritative report on a major issue, as by a team of experts; a government report outlining policy; or a short treatise whose purpose is to educate (contrast position paper) industry customers. It is called white paper because it was originally bound in white.
- The white ribbon is worn by movements denouncing violence against women and against queer youth. It is also worn by some feminists and was a symbol for peace in Quebec, in the beginning of 2003, as part of the popular opposition to war on Iraq.
- To "show the white feather" is to display cowardice. In cockfighting, a white feather in the tail is considered a mark of inferior breeding. In Victorian England a purported coward would be presented with a white feather.
- White is also one of the two opponents in many board games of abstract strategy, such as go, chess, and checkers.
- In both the French and Russian Revolutions, white symbolized royalism. Arthur Charles Fox-Davies has argued that white can be considered a tincture in heraldry separate from its use to represent argent, and in fact the labels borne on the arms, crests and supporters of members of the British Royal Family other than the reigning sovereign are invariably shown as white. The color is also used extensively by the Roman Catholic pope.

See also


- List of colors
- ja:白 simple:White

Color

Color or colour is the perception of the frequency (or wavelength) of light, and can be compared to how pitch (or a musical note) is the perception of the frequency or wavelength of sound. It is a perception which in humans derives from the ability of the fine structures of the eye to distinguish (usually three) differently filtered analyses of a view. The perception of color is influenced by biology (some people are born seeing colors differently or not at all; see color blindness), long-term history of the observer, and also by short-term effects such as the colors nearby. (This is the basis of many optical illusions.) The science of color is sometimes called chromatics. It includes the perception of color by the human eye, the origin of color in materials, color theory in art, and the physics of color in the electromagnetic spectrum.

Physics of color

The colors of the visible light spectrum.
color wavelength interval frequency interval
red ~ 625-740 nm ~ 480-405 THz
orange ~ 590-625 nm ~ 510-480 THz
yellow ~ 565-590 nm ~ 530-510 THz
green ~ 500-565 nm ~ 600-530 THz
cyan ~ 485-500 nm ~ 620-600 THz
blue ~ 440-485 nm ~ 680-620 THz
violet ~ 380-440 nm ~ 790-680 THz
Continuous optical spectrum
Image:Spectrum441pxWithnm.png
Designed for monitors with gamma 1.5.
Computer "spectrum"
Image:Computerspectrum.png
The bars below show the relative intensities of the three
colors mixed to make the color immediately above.
Color, frequency, and energy of light.
Color \lambda \,\!/nm \nu \,\!/1014 Hz \nu_b \,\!/104 cm-1 E \,\!/eV E \,\!/kJ mol-1
Infrared >1000 <3.00 <1.00 <1.24 <120
Red 700 4.28 1.43 1.77 171
Orange 620 4.84 1.61 2.00 193
Yellow 580 5.17 1.72 2.14 206
Green 530 5.66 1.89 2.34 226
Blue 470 6.38 2.13 2.64 254
Violet 420 7.14 2.38 2.95 285
Near ultraviolet 300 10.0 3.33 4.15 400
Far ultraviolet <200 >15.0 >5.00 >6.20 >598
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. 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). 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. 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.

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. 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.

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 something most computer screens are unable 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). 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. 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. See metamerism (color) for a basic introduction as to why color matching challenges exist.

Physical basis of color

A light wave can be analyzed as a superposition of sine waves, each of which has a specific frequency and wavelength. The eye gives limited information about the relative intensities of these sine waves (but not their phases — the eye is even more blind to phase than the ear, which can detect phase relationships of sounds only in certain very specific contexts). To understand which particular color perception will arise from a particular physical spectrum requires knowledge of the physiology of the retina. The human eye is also insensitive to polarization in most cases (though see Haidinger's brush), whereas some fish and mollusks can perceive it.

Color vision

Though the exact status of color is a matter of current philosophical dispute, color is arguably a psychophysical phenomenon that exists only in our minds. (See Qualia, for some of that dispute.) A "red" apple does not give off "red light", and it is misleading to think of things that we see, or of light itself, as objectively colored at all. Rather, the apple simply absorbs light of various wavelengths shining on it to different degrees, in such a way that the unabsorbed light which it reflects is perceived as red. An apple is perceived to be red only because normal human color vision perceives light with different mixes of wavelengths differently—and we have language to describe that difference. language In 1931, an international group of experts called the Commission Internationale d'Eclairage (CIE) developed a mathematical color model. The premise used by the CIE is that color is the combination of three things: a light source, an object, and an observer. The CIE tightly controlled each of these variables in an experiment that produced the measurements for the system. Although Aristotle and other ancient scientists speculated on the nature of light and color vision, it was not until Newton that light was correctly identified as the source of the color sensation. Goethe studied the theory of colors, and in 1801 Thomas Young proposed his trichromatic theory which was later refined by Hermann von Helmholtz. That theory was confirmed in the 1960s and will be described below. Hermann von Helmholtz The retina of the human eye contains three different types of color receptor cells, or cones. One type, relatively distinct from the other two, is most responsive to light that we perceive as violet, with wavelengths around 420 nm (cones of this type are sometimes called short-wavelength cones, S cones, or, most commonly but quite misleadingly, blue cones). The other two types are closely related genetically, chemically and in response. Each type is most responsive to light that we perceive as green or greenish. One of these types (sometimes called long-wavelength cones, L cones, or, misleadingly, red cones) is most sensitive to light we perceive as yellowish-green, with wavelengths around 564 nm; the other type (sometimes called middle-wavelength cones, M cones, or misleadingly green cones) is most sensitive to light perceived as green, with wavelengths around 534 nm. The term "red cones" for the long-wavelength cones is deprecated as this type is actually maximally responsive to light we perceive as greenish, albeit longer wavelength light than that which maximally excites the mid-wavelength/"green" cones. The sensitivity curves of the cones are roughly bell-shaped, and overlap considerably. The incoming signal spectrum is thus reduced by the eye to three values, sometimes called tristimulus values, representing the intensity of the response of each of the cone types. Because of the overlap between the sensitivity ranges, some combinations of responses in the three types of cone are impossible no matter what light stimulation is used. For example, it is not possible to stimulate only the mid-wavelength/"green" cones: the other cones must be stimulated to some degree at the same time, even if light of some single wavelength is used (including that to which the target cones are maximally sensitive). The set of all possible tristimulus values determines the human color space. It has been estimated that humans can distinguish roughly 10 million different colors, although the identification of a specific color is highly subjective, since even the two eyes of a single individual perceive colors slightly differently. This is discussed in more detail below. The rod system (which vision in very low light relies on exclusively) does not by itself sense differences in wavelength; therefore it is not normally implicated in color vision. But experiments have conclusively shown that in certain marginal conditions a combination of rod stimulation and cone stimulation can result in color discriminations not based on the mechanisms described above. While the mechanisms of color vision at the level of the cones in the retina are well described in terms of tristimulus values (see above), color processing and perception above that base level are organized differently. A dominant theory of the higher neural mechanisms of color vision proposes three opponent processes, or opponent channels, constructed out of the raw input from the cones: a red-green channel, a blue-yellow channel, and a black-white ("luminance") channel. This theory tries to account for the structure of our subjective color experience (see discussion below). Blue and yellow are considered complementary colors, or opposites: you could not experience a bluish yellow (or a greenish red), any more than you could experience a dark brightness or a hot coldness. The four "polar" colors proposed as extremes in the two opponent processes other than black-white have some natural claim to being called primary colors. This is in competition with various sets of three primary colors proposed as "generators" of all normal human color experience (see below).

Clinical issues

If one or more types of a person's color-sensing cones are missing or less responsive than normal to incoming light, that person has a smaller or skewed color space and is said to be color deficient. Another term frequently used is color blind, although this can be misleading; only a small fraction of color deficient individuals actually see completely in black and white, and most simply have anomalous color perception. Some kinds of color deficiency are caused by anomalies in the number or nature of cones of the various types, as just described. Others (like central or cortical achromatopsia) are caused by neural anomalies in those parts of the brain where visual processing takes place. Some animals may have more than three different types of color receptor (most marsupials, birds, reptiles, and fish; see tetrachromat, below) or fewer (most mammals; these are called dichromats and monochromats). Humans and other old-world primates are actually rather unusual in possessing three kinds of receptors. An unusual and elusive neurological condition sometimes affecting color perception is synaesthesia.

Tetrachromat

A normal human is a trichromat (from Greek: tri=three, chroma=color). In theory it may be possible for a person to have four, rather than three, distinct types of cone cell. If these four types are sufficiently distinct in spectral sensitivity and the neural processing of the input from the four types is developed, a person may be a tetrachromat (tetra=four). Such a person might have an extra and slightly different copy of either the medium- or long-wave cones. It is not clear whether such people exist or that the human brain could actually process the information from such an extra cone type separately from the standard three. However, strong evidence suggests that such people do exist, they are all female by genetic imperative, and their brains gladly adapt to use the additional information. For many species, tetrachromacy is the normal case, although the cone cells of animal tetrachromats have a very different (more evenly-spaced) spectral sensitivity distribution than those of possible human tetrachromats.

Color perception

There is an interesting phenomenon which occurs when an artist uses a limited color palette: the eye tends to compensate by seeing any grey or neutral color as the color which is missing from the color wheel. E.g.: in a limited palette consisting of red, yellow, black, and white, a mixture of yellow and black will appear as a variety of green, a mixture of red and black will appear as a variety of purple, and pure grey will appear bluish. When the eye shifts attention after viewing a color for some time, then an afterimage of the complement of that color (the color opposite to it in the color wheel) is perceived by the eye for some time wherever it moves. This effect of color perception was utilised by Vincent van Gogh, a Post-Impressionist painter.

Effect of luminosity

Note that the color experience of a given light mixture may vary with absolute luminosity, because both rods and cones are active at once in the eye, with each having different color curves, and rods taking over gradually from cones as the brightness of the scene is reduced. This effect leads to a change in color rendition with absolute illumination levels that can be summarised in the "Kruithof curve".

Cultural influences

Different cultures have different terms for colors, and may also assign some color names to slightly different parts of the spectrum, or have a different color ontology: for instance, the Han character 青 (pronounced qīng in Mandarin and aoi in Japanese) has a meaning that covers both blue and green; blue and green are traditionally considered shades of 青; In more contemporary terms, they are 藍 (lán) and 綠 (lǜ) respectively. Similarly, languages are selective when deciding which hues are split into different colors on the basis of how light or dark they are. Apart from the black-grey-white continuum, English splits some hues into several distinct colors according to lightness: such as red and pink or orange and brown. To English speakers, these pairs of colors, which are objectively no more different that light green and dark green, are conceived as totally different. An Italian will make the same red-pink and orange-brown distinctions, but will also make a further distinction between blu and azzurro, which English speakers would simply call dark and light blue. To Italian speakers, blu and azzurro are as separate as red and pink or orange and brown. Color terms evolve. It is argued that there are a limited number of universal "basic color terms" which begin to be used by individual cultures in a relatively fixed order. For example, a culture would start with only two terms, meaning roughly 'dark' (covering black, dark colors and cold colors such as blue ) and 'bright' (covering white, light colors and warm colors such as red), before adding more specific color names, in the order of red; green and/or yellow; blue; brown; and orange, pink, purple, and/or gray. Older arguments for this theory also stipulated that the acquisition and use of basic color terms further along the evolutionary order indicated a more complex culture with more highly developed technology. A somewhat dated example of a universal color categories theory is Basic Color Terms: Their Universality and Evolution (1969) by Brent Berlin and Paul Kay. A more recent example of a linguistic determinism theory might be Is color categorisation universal? New evidence from a stone-age culture (1999) by Jules Davidoff et al. The idea of linguistically determined color categories is often used as evidence for the Sapir-Whorf hypothesis (Language, Thought, and Reality (1956) by Benjamin Lee Whorf). Additionally, different colors are often associated with different emotional states, values, or groups, but these associations can vary between cultures. In one system, red is considered to motivate action; orange and purple are related to spirituality; yellow cheers; green creates cosiness and warmth; blue relaxes; and white is associated with either purity or death. These associations are described more fully in the individual color pages, and under color psychology. See also: National colors

Color constancy

The trichromatric theory discussed above is strictly true only if the whole scene seen by the eye is of one and the same color, which of course is unrealistic. In reality, the brain compares the various colors in a scene, in order to eliminate the effects of the illumination. If a scene is illuminated with one light, and then with another, as long as the difference between the light sources stays within a reasonable range, the colors of the scene will nevertheless appear constant to us. This was discovered by Edwin Land in the 1970s and led to his retinex theory of color constancy.

Contrast

Note: the following comparison requires an all-digital display setup (commonly, a laptop or DVI-connected LCD) to avoid errors caused by an unfortunate interaction between frequency response and gamma curves. Compare the visibility of the RGB primary and secondary colors against a white background:
red green blue red+green green+blue red+blue red+green+blue zero light
Again, compare variations on gray backgrounds—#7f7f7f, #5f5f5f & #9f9f9f—the eight RGB primaries are equidistant from #7f7f7f in a 3-d geometrical representation of RGB color space—a reminder of the importance of background color for color perception. Background = #7f7f7f
red green blue red+green green+blue red+blue red+green+blue zero light
And let's look at black again, for completeness. (Note that your monitor background probably is not perfectly black, as you can see by switching off the monitor.) Background = #000000
red green blue red+green green+blue red+blue red+green+blue zero light

Measurement and reproduction of color

monitor Two different light spectra which have the same effect on the three color receptors in the human eye will be perceived as the same color. This is exemplified by the white light that is emitted by fluorescent lamps, which typically has a spectrum consisting of a few narrow bands, while daylight has a continuous spectrum. The human eye cannot tell the difference between such light spectra just by looking into the light source, although reflected colors from objects can look different. (This is often exploited e.g. to make fruit or tomatoes look more brightly red in shops.) Similarly, most human color perceptions can be generated by a mixture of three colors called primaries. This is used to reproduce color scenes in photography, printing, television, and other media. There are a number of methods or color spaces for specifying a color in terms of three particular primary colors. Each method has its advantages and disadvantages depending on the particular application. No mixture of colors, though, can produce a fully pure color perceived as completely identical to a spectral color, although one can get very close for the longer wavelengths, where the chromaticity diagram above has a nearly straight edge. For example, mixing green light (530 nm) and blue light (460 nm) produces cyan light that is slightly desaturated, because response of the red color receptor would be greater to the green and blue light in the mixture than it would be to a pure cyan light at 485 nm that has the same intensity as the mixture of blue and green. Because of this, and because the primaries in color printing systems generally are not pure themselves, the colors reproduced are never perfectly saturated colors, and so spectral colors cannot be matched exactly. However, natural scenes rarely contain fully saturated colors, thus such scenes can usually be approximated well by these systems. The range of colors that can be reproduced with a given color reproduction system is called the gamut. The CIE chromaticity diagram can be used to describe the gamut. Another problem with color reproduction systems is connected with the acquisition devices, like cameras or scanners. The characteristics of the color sensors in the devices are often very far from the characteristics of the receptors in the human eye. In effect, acquisition of colors that have some special, often very "jagged", spectra caused for example by unusual lighting of the photographed scene can be relatively poor. Species that have color receptors different from humans, e. g. birds that may have four receptors, can differentiate some colors that look the same to a human. In such cases, a color reproduction system `tuned' to a human with normal color vision may give very inaccurate results for the other observers. The next problem is different color response of different devices. For color information stored and transferred in a digital form, color management technique based on color profiles attached to color data and to devices with different color response helps to avoid deformations of the reproduced colors. The technique works only for colors in gamut of the particular devices, e.g. it can still happen that your monitor is not able to show you real color of your goldfish even if your camera can receive and store the color information properly and vice versa.

Pigments and reflective media

When producing a color print or painting a surface, the applied paint changes the surface; if the surface is then illuminated with white light (which consists of equal intensities of all visible wavelengths), the reflected light will have a spectrum corresponding to the desired color. If a dab of paint looks red in white light, that is because the reflection of all non-red wavelengths is interrupted by the pigment, such that only red light is reflected into one's eye.

Structural color

Structural color is a property of some surfaces that are scored with fine parallel lines, formed of many thin parallel layers, or otherwise composed of periodic microstructures on the scale of the color's wavelength, to make a diffraction grating. The grating reflects some wavelengths more than others due to interference phenomena, causing white light to be reflected as colored light. Variations in the pattern's spacing often give rise to an iridescent effect, as seen in peacock feathers, films of oil, and mother of pearl, because the reflected color depends upon the viewing angle. Structural color is studied in the field of thin-film optics. A layman's term that describes particularly the most ordered structural colors is iridescence.

Footnotes

# The spelling color is predominant in American English, while colour is used in Commonwealth English. See our/or.

See also


- Metamerism
- Chromophore
- List of colors
- Qualia
- Color blindness
- Color temperature
- Color theory
  - Color scheme
- Colors and emblems for parties
- Political color
- Color psychology
- Synaesthesia (the mental connection, almost always arbitrary, between senses, usually involving color)
- Goethe's Theory of Colors
- The International Commission on Illumination defines colors and color spaces
- Thermochromics
- Tincture (heraldry). The colors in heraldry.

External links and sources


- [http://www.physicstoday.org/vol-55/iss-7/p43.html Comparative Article examining Goethean and Newtonian Color]
- [http://palimpsest.stanford.edu/waac/wn/wn21/wn21-3/wn21-308.html Kruithof curve citation]
- [http://www.soluxtli.com/edu13.htm Article by technical lighting manufacturer on rod/cone vision, with cites to literature]
- [http://www.angelfire.com/psy/reading/Colour.html The Psychology of Colour]
- [http://plato.stanford.edu/entries/color/ Stanford Encyclopedia of Philosophy entry]
- [http://webexhibits.org/causesofcolor/ Why are things colored?]
- [http://www.research.ibm.com/people/l/lloydt/color/color.HTM Why Should Engineers and Scientists Be Worried About Color?]
- [http://poynterextra.org/cp/colorproject/color.html Color, Contrast & Dimension in News Design] Category:Color Category:Image processing Category:Vision ko:색 ja:色 simple:Color

Visible spectrum

The visible spectrum is the portion of the optical spectrum (light or electromagnetic spectrum) that is visible to the human eye. There are no exact bounds to the optical spectrum, but there are 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. Wavelengths visible to the eye also pass through the "visible 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 than 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. flower into the colors of the optical spectrum.]]

Historical use of the term

Sir 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 that violet light as it passed through the prism, creating a spectrum of colors. It is now known light is composed of photons (which display some of the properties of a wave and some of the properties of a particle), 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 accorance with a dispersion relation: glass is one such material, which enables glass prisms to create an optical spectrum from white light.

Spectroscopy

dispersion relation transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible 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; and 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 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 motion of the stars analysed.

See also


- Frequency
- Rydberg formula
- Wavelength Category:Color Category:Electromagnetic spectrum
- Category:Vision ko:가시광선 ja:可視光線

Black

:This article is about the color black; for other uses, see Black (disambiguation). Black is a color with several subtle differences in meaning.

Color or light

Black can be defined as the visual impression experienced in directions from which no visible light reaches the eye. (This makes a contrast with whiteness, the impression of any combination of colors of light that equally stimulates all three types of color-sensitive visual receptors.) Pigments that absorb light rather than reflect it back to the eye "look black". A black pigment can, however, result from a combination of several pigments that collectively absorb all colors. If appropriate proportions of three primary pigments are mixed, the result reflects so little light as to be called "black". This provides two superficially opposite but actually complementary descriptions of black. Black is the lack of all colors of light, or an exhaustive combination of multiple colors of pigment. See also Primary colors and Primary pigments. In physics, a black body is a perfect absorber of light, but by a rule derived by Einstein it is also, when heated, the best emitter! Thus, the best radiative cooling, out of sunlight, is by using black paint, though it is important that it be black (a nearly perfect absorber) in the infrared as well. In elementary science far Ultraviolet light is called "black light" because, unseen per se, it causes many minerals and other substances to fluoresce.

Usage, symbolism, colloquial expressions

In the Western world, black is most often used with a negative connotation. The reasons for this are various, but the most widely accepted explanations are that night is experienced by humans as negative and dangerous. A secondary reason is that stains are most visible as dark additions to pale materials. In traditional class-based Western cultures "pale" skin indicated genteel domestic or intellectual indoor-work as opposed to rough outdoor labor in the fields. Aspects of this black/white opposition are not unique to the West, as, for example in the Indian varna system. African and African-American writers such as Frantz Fanon, Langston Hughes, Maya Angelou, and Ralph Ellison in particular identify a number of negative symbolisms surrounding the word "black", arguing that the good vs. bad dualism associated with white and black provide prejudiced connotations to color metaphors for race.
- A "black day", in these cultures, would refer to a sad or tragic day. The Romans already marked fasti days with white stones and nefasti days with black.
  - e.g. the Black September in Jordan refers to a month in which thousands were killed.
  - Black Monday, stock market crash on October 19, 1987
  - Black Tuesday, stock market crash on October 29, 1929 which is the start of the Great Depression.
  - Black Wednesday caused Britain to pull out of the ERM.
  - Black Thursday, stock market crash on October 24, 1929
  - Black Friday, various tragic events.
- many poems and songs use the word black negatively (e.g. "Paint It Black" (Rolling Stones), "Baby's In Black" (Beatles), "Black Eyed Dog" (Nick Drake).
- In these cultures, the color black is often used in painting, film, and literature to evoke a sense of the fear or to symbolize death. It has also been adopted as a symbolic color of the Halloween festival.
- In English heraldry, black means darkness, doubt, ignorance, and uncertainty. (The American Girls Handy Book, p. 370)
- Black is often a color of mourning. Historically, widows and widowers were expected to wear black for a year after the death of their spouses.
- Black comedy is a form of comedy dealing with morbid and serious topics.
- Black magic is an evil form of magic, often connected with death.
- A blacklist is a list of undesirable persons or entities.
- Evil witches are sterotypically dressed in black and good fairies in white. Melodrama villains are dressed in black and heroines in white dresses. In many Hollywood Westerns, bad cowboys wear black hats while the good ones wear white. Funeral dress is black, wedding gowns are white.
- In computer security, a blackhat is an attacker with evil intentions.
- The black market is illegal.
- Blackmail is illegal and is perceived as immoral.
- The black sheep of the family is the ne'er-do-well.
- The infamous "black hole of Calcutta."
- To blackball them is to block them from being admitted.
- Black thoughts are wicked ones.
- A black mood is a bad one (e.g. Winston Churchill's depression, which he called "my black dog").
- A black cat often means bad luck.
- If you sink the black eight-ball in billiards, you lose. (The ball with which you sink all others is the white cue ball.)
- A black mark against you is a bad thing.
- A black-hearted person is mean and unloving.
- Black propaganda is the use of known falsehoods, partial truths, or masquerades in propaganda to confuse an opponent. However, black can have positive symbolism.
- In the Maasai tribes of Kenya and Tanzania, the color black is associated with rain clouds, becoming a symbol of life and prosperity.
- In Western fashion, black is considered reliably stylish. This seems to be for reasons of contrast with the white skin (conversely, white t-shirts or suits are always stylish among African-Americans).
  - The colloquialism "the new black" is a reference to the latest trend or fad, on the basis that black is always fashionable.
- Black is seen as a color of seriousness and authority.
  - Many priests of the older religious denominations traditionally wear black.
  - The beltzak ("blacks" after their uniform) are the riot control units of the Basque Autonomous Police
- To say one's accounts are "in the black" is used to mean that one is free of debt.
  - (Being "in the red" is to be in debt—in traditional bookkeeping, negative amounts, such as costs, were printed in red ink, and positive amounts, like revenues, were printed in black ink, so that if "the bottom line" is printed in black, the firm is profiting.)
- The most sought-after rank in any martial art is a black belt.
- Cathar Perfects wore black (Cathars viewed black as a color of perfection).
- Dreaming of a black cat, or a black cat walking towards you, means good luck. Black can also be used in many neutral ways.
- The term black is often used in the West to denote race for persons whose skin color ranges from light to darker shades of brown. For a discussion of usage, see the main entry at Black (people) and Color metaphors for race.
- In arguments, things can be black-and-white, meaning that the issue at hand is dichotomized. However, this dualism is fraught with danger, as one may assign the colors "black and white" to bad and good, respectively.
- Black frequently symbolizes ambiguity, secrecy, and the unknown.
  - A black box is any device whose internal workings are unknown or irrelevant.
  - A black project is a secretive project, like Enigma Decryption, Narcotics, or police sting operations.
  - The blackshirts were Italian Fascist militias (negative for anti-fascists, but presumably positive for the original fascists themselves)
  - Some organizations are called "black" when they keep a low profile, like Sociétés Anonymes and secret societies.
- The term "black hole" is applied to collapsed stars. This term is metaphorical in the extreme, because few properties of black objects or black voids apply to black holes. However, light emitted within a black hole's event horizon cannot escape, hence a black hole cannot be directly observed.
- The national rugby team of New Zealand is called the All Blacks, in reference to their black outfits.
- Association football (soccer) referees traditionally wear all-black uniforms, however nowadays other uniform colors may also be worn.
- In auto racing, a black flag signals a certain driver to go into the pits.
- Black is also used for anarchist symbolism, sometimes split in diagonal with other colors for further symbolism. The plain black flag is explained as the opposite of a white flag signalling surrender. It is also sometimes an anarchist dress code, with a practical benefit of not attracting attention and making later identification of a subject difficult. This strategy referred to as a black bloc.
- In German politics 'black' is used colloquially to refer to the conservative parties CDU and CSU
- In ancient China, black was the symbol of North and Water, one of the main five colors. There is no negative or positive meaning associated with it.
- Black is the color of the snooker ball which has a 7-point value, and also the eighth billiard ball. In the game of eight ball, this ball is the ultimate object of the game, but, if accidentally sunk, means instant loss of the game.
- A polished black mirror is used for scrying, and is thought to help see into the paranormal world without interference or distraction.
- Members of the modern subculture of Goths dress predominantly in black.
- A large number of sports teams have uniforms designed with black colors - many feeling the color sometimes inparts a psychological advantage in its wearers. Among the more famous (or infamous) include the Oakland Raiders and Pittsburgh Steelers of the NFL, the San Antonio Spurs of the NBA, and Inter Milan of the Serie A of the Italian soccer leagues.

Black pigments


- Carbon black
- Ivory black
- Mars black

See also


- List of colors Category:Shades of gray Category:Spoken articles ja:黒 simple:Black

Light

Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength. The three basic dimensions of light (i.e., all electromagnetic radiation) are:
- Intensity (or brilliance or amplitude), which is related to the human perception of brightness of the light,
- Frequency (or wavelength), perceived by humans as the color of the light, and
- Polarization (or angle of vibration), which is not perceptible by humans under ordinary circumstances. Due to wave-particle duality, light simultaneously exhibits properties of both waves and particles. The precise nature of light is one of the key questions of modern physics.

Visible electromagnetic radiation

Visible light is the portion of the electromagnetic spectrum between the frequencies of 380 THz (3.8×1014 hertz) and 750 THz (7.5×1014 hertz). The speed (c), frequency (f or \nu), and wavelength (\lambda) of a wave obey the relation: : c = f~\lambda \,\! Because the speed of light in a vacuum is fixed, visible light can also be characterised by its wavelength of between 400 nanometres (abbreviated 'nm') and 800 nm (in a vacuum). Light entering the eye is absorbed by light-sensitive pigments within the rod cells and cone cells in the retina, triggering a cascade of events that creates electrical nerve impulses that travel through the optic nerve to the brain, producing vision.

Speed of light

Although some people speak of the "velocity of light", the word velocity should be reserved for vector quantities, that is, those with both magnitude and direction. The speed of light is a scalar quantity, having only magnitude and no direction, and therefore speed is the correct term. The speed of light has been measured many times, by many physicists. The best early measurement is Ole Rømer's (a Danish physicist), in 1676. By observing the motions of Jupiter and one of its moons, Io, with a telescope, and noting discrepancies in the apparent period of Io's orbit, Rømer calculated a speed of 227,000 kilometres per second (approximately 141,050 miles per second). The first successful measurement of the speed of light using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several thousand metres away, and placed a rotating cog wheel in the path of the beam from the source to the mirror and back again. At a certain rate of rotation, the beam could pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau measured the speed of light as 313,000 kilometres per second. Léon Foucault used rotating mirrors to obtain a value of 298,000 km/s (about 185,000 miles/s) in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's results in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 186,285 mile/s (299,796 km/s [1,079,265,600 km/h]). In daily use, the figures are rounded off to 300,000 km/s and 186,000 miles/s.

Refraction

All light propagates at a finite speed. Even moving observers always measure the same value of c, the speed of light in vacuum, as c = 299,792,458 metres per second (186,282.397 miles per second). When light passes through a transparent substance, such as air, water or glass, its speed is reduced, and it undergoes refraction. The reduction of the speed of light in a denser material can be indicated by the refractive index, n, which is defined as: : n = \frac \;\! Thus, n=1 in a vacuum and n>1 in matter. When a beam of light enters a medium from vacuum or another medium, it keeps the same frequency and changes its wavelength. If the incident beam is not orthogonal to the edge between the media, the direction of the beam will change. Refraction of light by lenses is used to focus light in magnifying glasses, spectacles and contact lenses, microscopes and refracting telescopes.

Optics

The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows offers many clues as to the nature of light as well as much enjoyment.

Color and wavelengths

The different wavelengths are detected by the human eye and then interpreted by the brain as colors, ranging from red at the longest wavelengths of about 700 nm. (lowest frequencies) to violet at the shortest wavelengths of about 400 nm. (highest frequencies). The intervening frequencies are seen as orange, yellow, green, cyan, blue, and, conventionally, indigo.
indigo
The wavelengths of the electromagnetic spectrum immediately outside the range that the human eye is able to perceive are called ultraviolet (UV) at the short wavelength (high frequency) end and infrared (IR) at the long wavelength (low frequency) end. Some animals, such as bees, can see UV radiation while others, such as pit viper snakes, can see infrared light. UV radiation is not normally directly perceived by humans except in a very delayed fashion, as overexposure of the skin to UV light can cause sunburn, or skin cancer, and underexposure can cause vitamin D deficiency. However, because UV is a higher frequency radiation than visible light, it very easily can cause materials to fluoresce visible light. Cameras that can detect IR and convert it to light are called, depending on their application, night-vision cameras or infrared cameras. These are different from image intensifier cameras, which only amplify available visible light. When intense radiation (of any frequency) is absorbed in the skin, it causes heating which can be felt. Since hot objects are strong sources of infrared radiation, IR radiation is commonly associated with this sensation. Any intense radiation that can be absorbed in the skin will have the same effect, however.

Measurement of light

The following quantities and units are used to measure the quantity or "brightness" of light. Light can also be characterised by:
- amplitude,
- color, wavelength, or frequency, and
- polarization (or angle of vibration).

Light sources

polarization There are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of black body radiation. Examples include sunlight (the radiation emitted by the chromosphere of the Sun at around 6,000 K peaks in the visible region of the electromagnetic spectrum), incandescent light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in flames. The peak of the blackbody spectrum is in the infrared for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue color as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colors can be seen when metal is heated to "red hot" or "white hot". The blue color is most commonly seen in a gas flame or a welder's torch. Atoms emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be be stimulated, as in a laser or a microwave maser. Acceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation. Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake. Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. This is used in fluorescent lights. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence. Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example of this. This mechanism is used in cathode ray tube televisions. Certain other mechanisms can produce light:
- scintillation
  - scintillator
- electroluminescence
- sonoluminescence
- triboluminescence
- radioactive decay
- particle-antiparticle annihilation

Theories about light

Early Greek ideas

In 55 BC Lucretius, continuing the ideas of earlier atomists, wrote that light and heat from the Sun were composed of minute particles. Ptolemy also wrote about the refraction of light.

10th century optical theory

The scientist Abu Ali al-Hasan ibn al-Haytham (965-c.1040), also known as Alhazen, developed a broad theory that explained vision, using geometry and anatomy, which stated that each point on an illuminated area or object radiates light rays in every direction, but that only one ray from each point, which strikes the eye perpendicularly, can be seen. The other rays strike at different angles and are not seen. He used the example of the pinhole camera, which produces an inverted image, to support his argument. Alhazen held light rays to be streams of minute particles that travelled at a finite speed. He improved Ptolemy's theory of the refraction of light. Alhazen's work did not become known in Europe until the late 16th century.

The 'plenum'

René Descartes (1596-1650) held that light was a disturbance of the plenum, the continuous substance of which the universe was composed. In 1637 he published a theory of the refraction of light which wrongly assumed that light travelled faster in a denser medium, by analogy with the behaviour of sound waves. Descartes' theory is often regarded as the forerunner of the wave theory of light.

Particle theory

Pierre Gassendi (1592-1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the plenum. He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether. Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to dominate physics during the 18th century.

Wave theory

In the 1660s, Robert Hooke published a wave theory of light. Christian Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the aether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium. The wave theory predicted that light waves could interfere with each other like sound waves (as noted in the 18th century by Thomas Young), and that light could be polarized. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colors were caused by different wavelengths of light, and explained color vision in terms of three-colored receptors in the eye. Another supporter of the wave theory was Euler. He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory. Later, Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Simeon Denis Poisson added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory. The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the luminiferous aether was proposed, but its existence was cast into strong doubt by the Michelson-Morley experiment. Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Léon Foucault, in 1850. His result supported the wave theory, and the classical particle theory was finally abandoned.

Electromagnetic theory

In 1845, Faraday discovered that the angle of polarisation of a beam of light as it passed through a polarising material could be altered by a magnetic field, an effect now known as Faraday rotation. This was the first evidence that light was related to electromagnetism. Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the aether. Faraday's work inspired James Clerk Maxwell to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: he first stated this result in 1862 in On Physical Lines of Force. In 1873, he published A Treatise on Electricity and Magnetism, which contained a full mathematical description of the behaviour of electric and magnetic fields, still known as Maxwell's equations. The technology of radio transmission was, and still is, based on this theory. The constant speed of light predicted by Maxwell's equations contradicted the mechanical laws of motion that had been unchallenged since the time of Galileo, which stated that all speeds were relative to the speed of the observer. A solution to this contradiction would later be found by Albert Einstein.

Particle theory revisited

The wave theory was accepted until the late 19th century, when Einstein described the photoelectric effect, by which light striking a surface caused electrons to change their momentum, which indicated a particle-like nature of light. This clearly contradicted the wave theory, and for years physicists tried in vain to resolve this contradiction.

Quantum theory

In 1900, Max Planck described quantum theory, in which light is considered to be as a particle that could exist in discrete amounts of energy only. These packets were called quanta, and the particle of light was given the name photon, to correspond with other particles being described around this time, such as the electron and proton. A photon has an energy, E, proportional to its frequency, f, by :E_f = hf = \frac \,\! where h is Planck's constant, \lambda is the wavelength and c is the speed of light. As it originally stood, this theory did not explain the simultaneous wave-like nature of light, though Planck would later work on theories that did. The Nobel Committee awarded Planck the Physics Prize in 1918 for his part in the founding of quantum theory.

Wave-particle duality

The modern theory that explains the nature of light is wave-particle duality, described by Albert Einstein in the early 1900s, based on his work on the photoelectric effect and Planck's results. Einstein determined that the energy of a photon is proportional to its frequency. More generally, the theory states that everything has both a particle nature and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned if an object has a large mass, so it took until an experiment by Louis de Broglie in 1924 to realise that electrons also exhibited wave-particle duality. Einstein received the Nobel Prize in 1921 for his work with the wave-particle duality on photons, and de Broglie followed in 1929 for his extension to other particles.

A light wave

1929 that oscillate perpendicular to each other and to the direction of motion (a transverse wave).]] The electric and magnetic fields are perpendicular to the direction of travel and to each other. This picture depicts a very special case, linearly polarized light. See Polarization for a description of the general case and an explanation of linear polarization. While these relations of the electric and magnetic fields are always true, the subtle difference in the general case is that the direction and amplitude of the magnetic (or electric) field can vary, in one place, with time, or, in one instant, can vary along the direction of propagation.

See also


- Color temperature
- Huygens' principle
- Fermat's principle
- International Commission on Illumination
- Light pollution
- Lighting
- Photic sneeze reflex
- Photometry
- Spectrometry Category:Optics Category:Image processing ko:빛 ms:Cahaya ja:光 simple:Light th:แสง

Primary color

:This page is about actual colors. For the political book and movie, see Primary Colors A primary color (or colour) is a color that cannot be created by mixing other colors in the gamut of a given color space. Primary colors may themselves be mixed to produce most of the colors in a given color space: mixing two primary colors produces what is generally called a secondary color, mixing a secondary with a primary produces what is sometimes called a tertiary color. Traditionally, the colors red, yellow, and blue are considered to be primary pigments in the art world. However those colors are not the same hue as the "red", "yellow" and "blue" used in alternate color systems. Many modern applications use primary additive colors of red, green and blue; and the primary pigments of magenta, yellow, and cyan. If the color space is considered as a vector space, the primary colors can be regarded as a set of basis vectors for that space.

Biological basis

Primary colors are not a physical but rather a biological concept, based on the physiological response of the human eye to light. The human eye contains receptors called cones which normally respond to red, green, and blue light. Humans and other species with three such types of color receptors are known as trichromats. Although the peak responsivities of the cones do not occur exactly at the red, green and blue frequencies, those three colors are chosen as primary because they provide a wide gamut, making it possible to almost independently stimulate the three color receptors. To generate optimal color ranges for species other than humans, other additive primary colors would have to be used. For species known as tetrachromats with four different color receptors, one would use four primary colors. Many birds and marsupials are tetrachromats and it has been suggested that some female humans are born as tetrachromats as well, having an extra receptor for yellow. On the other hand, most mammals have only two types of color receptors and are therefore dichromats; to them, there are only two primary colors.

Additive primaries

dichromat Media that combine emitted lights to create the sensation of a range of colors are using the additive color system. Television is the most common use of this. The Additive primaries are red, green, and blue. Because of the response curves of the three different color receptors in the human eye, these colors are optimal in the sense that the largest range of colors (gamut) visible by humans can be generated by mixing light of these colors. Additive mixing of red and green light, produce shades of yellow or orange. Mixing green and blue produces shades of cyan, and mixing red and blue produces shades of purple and magenta. Mixing equal proportions of the additive primaries results in shades of grey; when all three colors are fully saturated, the result is white. The color space that is generated is called the RGB ("red, green, blue") color space

Subtractive primaries

Media that use reflected light and colorants to produce colors are using the subtractive color method of color mixing. In the printing industry, to produce the varying colors, apply the subtractive primaries yellow, cyan, and magenta together in varying amounts. Subtractive color works best when the surface (or paper) is white, or close to it. magenta Mixing yellow and cyan produces shades of green; mixing yellow with magenta produces shades of red, and mixing magenta with cyan produces shades of blue. In theory, mixing equal amounts of all three pigments should produce shades of grey, resulting in black when all three are fully saturated, but in practice they tend to produce muddy brown colors. For this reason, a fourth "primary" pigment, black, is often used in addition to the cyan, magenta, and yellow colors. The color space generated is the so-called CMYK color space. (standing for "Cyan, Magenta, Yellow, and Black - K is used to represent black as 'B' could be confused with 'Blue'"). In practice, mixtures of actual materials like paint tend to be less precise. Brighter, or more specific colors can be created using natural pigments instead of mixing, and natural properties of pigments can interfere with the mixing. For example, mixing magenta and green in acrylic creates a dark cyan - something which would not happen if the mixing process were perfectly subtractive. In the subtractive model, adding white to a color does not change its hue but does reduce its saturation. For a more detailed and extensive treatment of color, see color. See printing

See Also


- Secondary color
- Tertiary color

External links


- [http://www.newton.dep.anl.gov/askasci/phy00/phy00871.htm Ask A Scientist: Primary Colors]
- [http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colcon.html#c1 The Color-Sensitive Cones at HyperPhysics]
- [http://handprint.com/HP/WCL/wcolor.html Handprint.com : do "primary" colors exist?] - a very comprehensive site on color primaries, color perception, color psychology, color theory, and color mixing
- [http://www.cecs.csulb.edu/~jewett/colors/index.html Color Tutorial] Category:Color

Optical spectrum

The visible spectrum is the portion of the optical spectrum (light or electromagnetic spectrum) that is visible to the human eye. There are no exact bounds to the optical spectrum, but there are 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. Wavelengths visible to the eye also pass through the "visible 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 than 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. flower into the colors of the optical spectrum.]]

Historical use of the term

Sir 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 that violet light as it passed through the prism, creating a spectrum of colors. It is now known light is composed of photons (which display some of the properties of a wave and some of the properties of a particle), 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 accorance with a dispersion relation: glass is one such material, which enables glass prisms to create an optical spectrum from white light.

Spectroscopy

dispersion relation transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible 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; and 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 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 motion of the stars analysed.

See also


- Frequency
- Rydberg formula
- Wavelength Category:Color Category:Electromagnetic spectrum
- Category:Vision ko:가시광선 ja:可視光線

Red

Red is a color at the lowest frequencies of light discernible by the human eye. Red light has a wavelength range of roughly 630-760 nm. Lower frequencies are called infrared, or far red. Red is an additive primary color, complementary to cyan. It was once considered to be a subtractive primary color, and is still sometimes described as such in non-scientific literature; however, the colors cyan, magenta and yellow are now known to be closer to the true subtractive primary colors detected by the eye, and are used in modern color printing.

Usage, symbolism, colloquial expressions


- Red catches people's attention, and is often used to indicate danger or emergency.
  - Red is the color of heat and fire. Taps for hot water are often labeled red. Red is commonly the color of fire alarm boxes, fire extinguishers, and the firefighter profession itself.
  - Red denotes "stop" in, for instance, stop signs, traffic signals, brake lights, or the flashing lights of a school bus.
  - A Red Cross, Red Crescent or Red Crystal flag signify medical personnel, facilities, or equipment, or the Geneva conventions.
  - Red indicates extreme danger on Western color-coded scales, such as wildfire hazard signs or the U.S. Homeland Security Advisory System.
  - In auto racing, a red flag signals all cars to immediately stop. The redline is the maximum speed an engine and its components can run.
  - Emergency exits on passenger aircraft are indicated by red signs and lighting.
  - "Redlining" is delineating a forbidden area (as on a map), for instance where a company denies or increases the cost of services, and is illegal in various circumstances in the U.S.
  - With ships on collision courses, the ship on