Red

The human eye sees red when it looks at light with a wavelength between approximately 625 and 740 nanometers.[1] It is a primary color in the RGB color model and the light just past this range is called infrared, or below red, and cannot be seen by human eyes, although it can be sensed as heat.[5] In the language of optics, red is the color evoked by light that stimulates neither the S or the M (short and medium wavelength) cone cells of the retina, combined with a fading stimulation of the L (long-wavelength) cone cells.[6]

Primates can distinguish the full range of the colors of the spectrum visible to humans, but many kinds of mammals, such as dogs and cattle, have dichromacy, which means they can see blues and yellows, but cannot distinguish red and green (both are seen as gray). Bulls, for instance, cannot see the red color of the cape of a bullfighter, but they are agitated by its movement.[7] (See color vision).

One theory for why primates developed sensitivity to red is that it allowed ripe fruit to be distinguished from unripe fruit and inedible vegetation.[8] This may have driven further adaptations by species taking advantage of this new ability, such as the emergence of red faces.[9]

Red light is used to help adapt night vision in low-light or night time, as the rod cells in the human eye are not sensitive to red.[10][11]

Red illumination was (and sometimes still is) used as a safelight while working in a darkroom as it does not expose most photographic paper and some films.[12] Today modern darkrooms usually use an amber safelight.[citation needed]

In color theory and on a computer screen

On the color wheel long used by painters, and in traditional color theory, red is one of the three primary colors, along with blue and yellow. Painters in the Renaissance mixed red and blue to make violet: Cennino Cennini, in his 15th-century manual on painting, wrote, "If you want to make a lovely violet colour, take fine lac [red lake], ultramarine blue (the same amount of the one as of the other) with a binder"; he noted that it could also be made by mixing blue indigo and red hematite.[13]

In modern color theory, also known as the RGB color model, red, green and blue are additive primary colors. Red, green and blue light combined makes white light, and these three colors, combined in different mixtures, can produce nearly any other color. This is the principle that is used to make all of the colors on your computer screen and your television. For example, magenta on a computer screen is made by a similar formula to that used by Cennino Cennini in the Renaissance to make violet, but using additive colors and light instead of pigment: it is created by combining red and blue light at equal intensity on a black screen. Violet is made on a computer screen in a similar way, but with a greater amount of blue light and less red light.[14]

(See Web colors and RGB color model)

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  In a traditional color wheel from 1708, red, yellow and blue are primary colors. Red and yellow make orange; red and blue make violet.
  

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  In modern color theory, red, green and blue are the additive primary colors, and together they make white. A combination of red, green and blue light in varying proportions makes all the colors on your computer screen and television screen.
  

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  Tiny Red, green and blue sub-pixels (enlarged on left side of image) create the colors you see on your computer screen and TV.
  

Color of sunset

As a ray of white sunlight travels through the atmosphere to the eye, some of the colors are scattered out of the beam by air molecules and airborne particles due to Rayleigh scattering, changing the final color of the beam that is seen. Colors with a shorter wavelength, such as blue and green, scatter more strongly, and are removed from the light that finally reaches the eye.[15]
At sunrise and sunset, when the path of the sunlight through the atmosphere to the eye is longest, the blue and green components are removed almost completely, leaving the longer wavelength orange and red light. The remaining reddened sunlight can also be scattered by cloud droplets and other relatively large particles, which give the sky above the horizon its red glow.[16]

Lasers

Lasers emitting in the red region of the spectrum have been available since the invention of the ruby laser in 1960. In 1962 the red helium–neon laser was invented,[17] and these two types of lasers were widely used in many scientific applications including holography, and in education. Red helium–neon lasers were used commercially in LaserDisc players. The use of red laser diodes became widespread with the commercial success of modern DVD players, which use a 660 nm laser diode technology. Today, red and red-orange laser diodes are widely available to the public in the form of extremely inexpensive laser pointers. Portable, high-powered versions are also available for various applications.[18] More recently, 671 nm diode-pumped solid state (DPSS) lasers have been introduced to the market for all-DPSS laser display systems, particle image velocimetry, Raman spectroscopy, and holography.[19]

Red's wavelength has been an important factor in laser technologies; red lasers, used in early compact disc technologies, are being replaced by blue lasers, as red's longer wavelength causes the laser's recordings to take up more space on the disc than would blue-laser recordings.[20]

Astronomy

• Mars is called the Red Planet because of the reddish color imparted to its surface by the abundant iron oxide present there.[21]


• Astronomical objects that are moving away from the observer exhibit a Doppler red shift.


• Jupiter's surface displays a Great Red Spot caused by an oval-shaped mega storm south of the planet's equator.[22]


• Red giants are stars that have exhausted the supply of hydrogen in their cores and switched to thermonuclear fusion of hydrogen in a shell that surrounds its core. They have radii tens to hundreds of times larger than that of the Sun. However, their outer envelope is much lower in temperature, giving them an orange hue. Despite the lower energy density of their envelope, red giants are many times more luminous than the Sun due to their large size.


• Red supergiants like Antares, Betelgeuse, VY Canis Majoris and UY Scuti, one of the biggest stars in the Universe, are the biggest variety of red giants. They are huge in size, with radii 200 to 2600 times greater than our Sun, but relatively cool in temperature (3000–4500 K), causing their distinct red tint. Because they are shrinking rapidly in size, they are surrounded by an envelope or skin much bigger than the star itself. The envelope of Betelgeuse is 250 times bigger than the star inside.


• A red dwarf is a small and relatively cool star, which has a mass of less than half that of the Sun and a surface temperature of less than 4,000 K. Red dwarfs are by far the most common type of star in the Galaxy, but due to their low luminosity, from Earth, none are visible to the naked eye.[23]

Pigments and dyes

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  Red ochre cliffs near Roussillon in France. Red ochre is composed of clay tinted with hematite. Ochre was the first pigment used by man in prehistoric cave paintings.
  

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  Vermilion pigment, made from cinnabar. This was the pigment used in the murals of Pompeii and to color Chinese lacquerware beginning in the Song dynasty.
  

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  Despite its yellow greenish flower, the roots of the Rubia tinctorum, or madder plant, produced the most common red dye used from ancient times until the 19th century.
  

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  Red lead, also known as minium, has been used since the time of the ancient Greeks. Chemically it is known as lead tetroxide. The Romans prepared it by the roasting of lead white pigment. It was commonly used in the Middle Ages for the headings and decoration of illuminated manuscripts.
  

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  Alizarin was the first synthetic red dye, created by German chemists in 1868. It duplicated the colorant in the madder plant, but was cheaper and longer lasting. After its introduction, the production of natural dyes from the madder plant virtually ceased.
  

Red lac, red lake and crimson lake

Red lac, also called red lake, crimson lake or carmine lake, was an important red pigment in Renaissance and Baroque art. Since it was translucent, thin layers of red lac were built up or glazed over a more opaque dark color to create a particularly deep and vivid color.

Unlike vermilion or red ochre, made from minerals, red lake pigments are made by mixing organic dyes, made from insects or plants, with white chalk or alum. Red lac was made from the gum lac, the dark red resinous substance secreted by various scale insects, particularly the Laccifer lacca from India.[24] Carmine lake was made from the cochineal insect from Central and South America, Kermes lake came from a different scale insect, kermes vermilio, which thrived on oak trees around the Mediterranean. Other red lakes were made from the rose madder plant and from the brazilwood tree.

Red lake pigments were an important part of the palette of 16th-century Venetian painters, particularly Titian, but they were used in all periods.[25] Since the red lakes were made from organic dyes, they tended to be fugitive, becoming unstable and fading when exposed to sunlight.

Food coloring

The most common synthetic food coloring today is Allura Red AC, a red azo dye that goes by several names including: Allura Red, Food Red 17, C.I. 16035, FD&C Red 40,[26][27] It was originally manufactured from coal tar, but now is mostly made from petroleum.[citation needed]

In Europe, Allura Red AC is not recommended for consumption by children. It is banned in Denmark, Belgium, France and Switzerland, and was also banned in Sweden until the country joined the European Union in 1994.[28] The European Union approves Allura Red AC as a food colorant, but EU countries' local laws banning food colorants are preserved.[29]

In the United States, Allura Red AC is approved by the Food and Drug Administration (FDA) for use in cosmetics, drugs, and food. It is used in some tattoo inks and is used in many products, such as soft drinks, children's medications, and cotton candy. On June 30, 2010, the Center for Science in the Public Interest (CSPI) called for the FDA to ban Red 40.[30]

Because of public concerns about possible health risks associated with synthetic dyes, many companies have switched to using natural pigments such as carmine, made from crushing the tiny female cochineal insect. This insect, originating in Mexico and Central America, was used to make the brilliant scarlet dyes of the European Renaissance.[citation needed]

Autumn leaves

The red of autumn leaves is produced by pigments called anthocyanins. They are not present in the leaf throughout the growing season, but are actively produced towards the end of summer.[31] They develop in late summer in the sap of the cells of the leaf, and this development is the result of complex interactions of many influences—both inside and outside the plant. Their formation depends on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaf is reduced.[32]

During the summer growing season, phosphate is at a high level. It has a vital role in the breakdown of the sugars manufactured by chlorophyll. But in the fall, phosphate, along with the other chemicals and nutrients, moves out of the leaf into the stem of the plant. When this happens, the sugar-breakdown process changes, leading to the production of anthocyanin pigments. The brighter the light during this period, the greater the production of anthocyanins and the more brilliant the resulting color display. When the days of autumn are bright and cool, and the nights are chilly but not freezing, the brightest colorations usually develop.

Anthocyanins temporarily color the edges of some of the very young leaves as they unfold from the buds in early spring. They also give the familiar color to such common fruits as cranberries, red apples, blueberries, cherries, raspberries, and plums.

Anthocyanins are present in about 10% of tree species in temperate regions, although in certain areas—a famous example being New England—up to 70% of tree species may produce the pigment.[31] In autumn forests they appear vivid in the maples, oaks, sourwood, sweetgums, dogwoods, tupelos, cherry trees and persimmons. These same pigments often combine with the carotenoids' colors to create the deeper orange, fiery reds, and bronzes typical of many hardwood species. (See Autumn leaf color).

Blood and other reds in nature

Oxygenated blood is red due to the presence of oxygenated hemoglobin that contains iron molecules, with the iron components reflecting red light.[33][34] Red meat gets its color from the iron found in the myoglobin and hemoglobin in the muscles and residual blood.[35]

Plants like apples, strawberries, cherries, tomatoes, peppers, and pomegranates are often colored by forms of carotenoids, red pigments that also assist photosynthesis.[36]

• When used to describe natural animal coloration, "red" usually refers to a brownish, reddish-brown or ginger color. In this sense it is used to describe coat colors of reddish-brown cattle and dogs, and in the names of various animal species or breeds such as red fox, red squirrel, red deer, European robin, red grouse, red knot, redstart, redwing, red setter, Red Devon cattle, etc. This reddish-brown color is also meant when using the terms red ochre and red hair.


• The red herring dragged across a trail to destroy the scent gets its color from the heavy salting and slow smoking of the fish, which results in a warm, brown color.


• When used for flowers, red often refers to purplish (red deadnettle, red clover, red helleborine) or pink (red campion, red valerian) colors.

Hair color

Red hair occurs naturally on approximately 1–2% of the human population.[37] It occurs more frequently (2–6%) in people of northern or western European ancestry, and less frequently in other populations. Red hair appears in people with two copies of a recessive gene on chromosome 16 which causes a mutation in the MC1R protein.[38]

Red hair varies from a deep burgundy through burnt orange to bright copper. It is characterized by high levels of the reddish pigment pheomelanin (which also accounts for the red color of the lips) and relatively low levels of the dark pigment eumelanin. The term redhead (originally redd hede) has been in use since at least 1510.[39] Cultural reactions have varied from ridicule to admiration; many common stereotypes exist regarding redheads and they are often portrayed as fiery-tempered.[citation needed]

In animal and human behavior

Red is associated with dominance in a number of animal species.[40] For example, in mandrills, red coloration of the face is greatest in alpha males, increasingly less prominent in lower ranking subordinates, and directly correlated with levels of testosterone.[41] Red can also affect the perception of dominance by others, leading to significant differences in mortality, reproductive success and parental investment between individuals displaying red and those not.[42] In humans, wearing red has been linked with increased performance in competitions, including professional sport[43][44] and multiplayer video games.[45] Controlled tests have demonstrated that wearing red does not increase performance or levels of testosterone during exercise, so the effect is likely to be produced by perceived rather than actual performance.[46] Judges of tae kwon do have been shown to favor competitors wearing red protective gear over blue,[47] and, when asked, a significant majority of people say that red abstract shapes are more "dominant", "aggressive", and "likely to win a physical competition" than blue shapes.[40] In contrast to its positive effect in physical competition and dominance behavior, exposure to red decreases performance in cognitive tasks[48] and elicits aversion in psychological tests where subjects are placed in an "achievement" context (e.g. taking an IQ test).[49]

Inside cave 13B at Pinnacle Point, an archeological site found on the coast of South Africa, paleoanthropologists in 2000 found evidence that, between 170,000 and 40,000 years ago, Late Stone Age people were scraping and grinding ochre, a clay colored red by iron oxide, probably with the intention of using it to color their bodies.[50]

Red hematite powder was also found scattered around the remains at a grave site in a Zhoukoudian cave complex near Beijing. The site has evidence of habitation as early as 700,000 years ago. The hematite might have been used to symbolize blood in an offering to the dead.[3]:4

In ancient Egypt, red was associated with life, health, and victory. Egyptians would color themselves with red ochre during celebrations.[51] Egyptian women used red ochre as a cosmetic to redden cheeks and lips[52] and also used henna to color their hair and paint their nails.[53]

In Renaissance painting, red was used to draw the attention of the viewer; it was often used as the color of the cloak or costume of Christ, the Virgin Mary, or another central figure. In Venice, Titian was the master of fine reds, particularly vermilion; he used many layers of pigment mixed with a semi-transparent glaze, which let the light pass through, to create a more luminous color.

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  The Assumption, by Titian (1516–18). The figures of God, the Virgin Mary and two apostles are highlighted by their vermilion red costumes.
  

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  The young Queen Elizabeth I (here in about 1563) liked to wear bright reds, before she adopted the more sober image of the "Virgin Queen". Her satin gown was probably dyed with kermes.
  

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  The Wedding Dance (1566), by Pieter Bruegel the Elder. In Renaissance Flanders, people of all social classes wore red at celebrations. The dye came from the root of the madder plant, which tended toward orange.
  

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  Woman with a wine glass, by Johannes Vermeer (1659–60). Vermeer used different shades and tints of vermilion to paint the red skirt, then glazed it with madder lake to make a more luminous color.
  

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  Dyed feather headdress from the Aztec people of Mexico and Central America. For red they used cochineal, a brilliant scarlet dye made from insects.
  

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  A native of Central America collecting cochineal insects from a cactus to make red dye (1777). From the 16th until the 19th century, it was a highly profitable export from Spanish Mexico to Europe.
  

During the French Revolution, the Jacobins and other more radical parties adopted the red flag; it was taken from red flags hoisted by the French government to declare a state of siege or emergency. Many of them wore a red Phrygian cap, or liberty cap, modeled after the caps worn by freed slaves in Ancient Rome. During the height of the Reign of Terror, Women wearing red caps gathered around the guillotine to celebrate each execution. They were called the "Furies of the guillotine". The guillotines used during the Reign of Terror in 1792 and 1793 were painted red, or made of red wood. During the Reign of Terror a statue of a woman titled liberty, painted red, was placed in the square in front of the guillotine. After the end of the Reign of Terror, France went back to the blue, white and red tricolor, whose red was taken from the red and blue colors of the city of Paris, and was the traditional color of Saint Denis, the Christian martyr and patron saint of Paris.

In the 20th century, red was the color of Revolution; it was the color of the Bolshevik Revolution in 1917 and of the Chinese Revolution of 1949, and later of the Cultural Revolution. Red was the color of communist parties from Eastern Europe to Cuba to Vietnam.

In the late 19th and early 20th century, the German chemical industry invented two new synthetic red pigments: cadmium red, which was the color of natural vermilion, and mars red, which was a synthetic red ochre, the color of the very first natural red pigment.