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rhodopsin

[roh-dop-sin]

or visual purple

Light-sensitive, purple-red organic pigment contained in the rod cells of the retina that allows the eye to see in black and white in dim light. It is composed of opsin, a protein, linked to retinal, a conjugated molecule (see conjugation) formed from vitamin A. Photons of light that enter the eye are absorbed by retinal and cause it to change its configuration, starting a biochemical chain of events that ends with impulses being sent along the optic nerve to the brain. In bright light, to protect rod cells from overstimulation, rhodopsin breaks down into retinal and opsin, both of which are colourless. In dim light or darkness the process is reversed (dark adaptation), and purple-red rhodopsin is reformed. Similar light-sensitive compounds made of retinal and other opsin proteins are the pigments in the retina's cone cells responsible for colour vision in bright light.

Learn more about rhodopsin with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Purple

Purple is a general term for the range of shades of color occurring between red and blue. It is formed (in both subtractive pigment and additive light combinations) by mixing the primary colors red and blue in varying proportions, with possibly a very small quantity of the third primary color (green for light or yellow for pigment). There is disagreement over exactly which shades can be described as purple, some people preferring more precise terms such as magenta or heliotrope for particular shades. A difference in retinal sensitivity to red and blue light between individuals can cause further disagreement.

In color theory, a 'purple' is defined as any non-spectral color between violet and red. The spectral color violet is not a purple.

In art, purple is the color on the color wheel between magenta and violet and its tints and shades. This color, electric purple, is shown below.

In human color psychology, purple is associated with royalty, regality, and nobility (stemming from its use in heraldry to denote gentry).

Etymology and definitions

The word 'purple' comes from the Old English word purpul which originates from the Latin purpura. This in turn is derived from the Koine Greek πορφύρα (porphyra), name of the dye manufactured in Classical antiquity from the mucus-secretion of the hypobranchial gland of a marine snail known as the Murex brandaris or the spiny dye-murex.

The first recorded use of the word 'purple' in English was in the year AD 975.

Purple versus violet

Violet is a spectral color (approximately 420–380nm), of a shorter wavelength than blue, while purple is a combination of red and blue or violet light. The purples are colors that are not spectral colors – purples are extra-spectral colors. In fact, purple was not present on Newton's color wheel (which went directly from violet to red), though it is present on modern ones, between red and violet. There is no such thing as the "wavelength of purple light"; it only exists as a combination.

On the CIE xy chromaticity diagram, violet is on the curved edge in the lower left, while purples are the straight line connecting the extreme colors red and violet.

One interesting psychophysical feature of the two colors that can be used to separate them is their appearance with increase of light intensity. Violet, as light intensity increases, appears to take on a far more blue hue as a result of what is known as the Bezold-Brücke shift. The same increase in blueness is not noted in purples.

Violet cannot be reproduced by a Red-Green-Blue (RGB) color system, and must be simulated by a mixture of red and blue (purple). The shade of violet simulated in the color box above is just over halfway between magenta and blue on the color wheel.

Properties

On a chromaticity diagram, the straight line connecting the extreme spectral colors (red and violet) is known as the 'line of purples' (or 'purple boundary'); it represents one limit of human color perception. The color magenta used in the CMYK printing process is on the line of purples, but most people associate the term "purple" with a somewhat bluer shade. Some common confusion exists concerning the color names "purple" and "violet". Purple is a mixture of red and blue light, whereas violet is a spectral color.

Historical development of purple

Tyrian purple: Classical antiquity

The actual color of Tyrian purple, the original color purple from which the name purple is derived, is the color of a dye made from a mollusk that, because of its incredible expense (many times more expensive than gold), in classical antiquity became a symbol of royalty because only the very wealthy could afford it. Therefore, Tyrian purple is also called imperial purple.

Tyrian purple may have been discovered as early as the time of the Minoan civilization. Alexander the Great (when giving imperial audiences as the emperor of the Macedonian Empire), the emperors of the Seleucid Empire, and the kings of Ptolemaic Egypt wore Tyrian purple. The imperial robes of Roman emperors were Tyrian purple trimmed in metallic gold thread. The badge of office of a Roman Senator was a stripe of Tyrian purple on their white toga. Tyrian purple was continued in use by the emperors of the Eastern Roman Empire until its final collapse in 1453.

Han purple: Ancient China

Han purple is a type of artificial pigment found in China between 500 BC and AD 220. It was used in the decoration of the Xian Terracotta Army.

Han purple is a purple in the sense that the term is used in colloquial English, i.e., it is a color between red and blue; however, it is not a purple in the sense that the term is used in color theory, i.e. a non-spectral color between red and violet on the line of purples on the CIE chromaticity diagram.

Royal purple: Medieval Europe

This shade of purple is bluer than the ancient Tyrian purple.

In medieval Europe, blue dyes were rare and expensive, so only the most wealthy or the aristocracy could afford to wear them. (The working class wore mainly green and brown.) Because of this (and also because Tyrian purple had gone out of use in western Europe after the collapse of the Western Roman Empire in AD 476), Europeans' idea of purple shifted towards this more bluish purple known as royal purple because of its similarity to the royal blue worn by the aristocracy. This was the shade of purple worn by kings in medieval Europe.

Artists pigment purple (red-violet): 1930s

'Royal purple' (shown above) or the dark violet color known as vulgar purple is the common layman's idea of purple, but professional artists, following Munsell color system (introduced in 1905 and widely accepted by 1930), regard purple as being synonymous with the red-violet color shown at right, in order to clearly distinguish purple from violet and thus have access to a larger palette of colors. This red-violet color, called artist's purple by artists, is the pigment color that would be on a pigment color color wheel between pigment violet and pigment (process) magenta. In the Munsell color system, this color at the maximum chroma of 12 is called Red-Purple.

Artists pigments and colored pencils labeled as purple are colored the red-violet color shown at right.

Electric purple: 2000s

This color, electric purple, is precisely halfway between violet and magenta and thus fits the artistic definition of purple.

Using additive colors such as those on computer screens, it is possible to create a much brighter purple than with pigments where the mixing subtracts frequencies from the component primary colors. The equivalent color on a computer to the pigment color red-violet shown above would be this electric purple, i.e. the much brighter purple you can see reproduced on the screen of an electronic computer. This color is pure purple conceived as computer artists conceive it, as the pure chroma on the computer screen color wheel halfway between electric violet and electric magenta. Thus, electric purple is the purest and brightest purple that it is possible to display on a computer screen.

Computer web color purples

Purple (HTML/CSS color)

This purple used in HTML and CSS actually is deeper and has a more reddish hue (#800080) than the X11 color purple shown below as purple (X11 color) (#A020F0), which is bluer and brighter.

This color may be called HTML/CSS purple.

Purple (X11 color)

At right is displayed the color purple, as defined in the X11 color, which is a lot brighter and bluer than the HTML purple shown above.

See the chart Color names that clash between X11 and HTML/CSS in the X11 color names article to see those colors which are different in HTML and X11.

This color can be called X11 purple.

Medium purple (X11)

Displayed at right is the web color medium purple.

This color is a medium shade of the bright X11 purple shown above.

Additional variations of purple

Orchid

The color orchid is a light shade of purple. The name 'orchid' originates from the flowers of some species of the vast orchid flower family, such as Laelia furfuracea and Ascocentrum pusillum, which have petals of this color.

Heliotrope

The color heliotrope is a brilliant shade of purple.

Heliotrope is a pink-purple tint that is a representation of the color of the heliotrope flower.

Psychedelic purple

The pure essence of purple was approximated in pigment in the late 1960s by mixing fluorescent magenta and fluorescent blue pigments together to make fluorescent purple to use in psychedelic black light paintings. This shade of purple was very popular among hippies and was the favorite color of Jimi Hendrix, therefore it is called psychedelic purple. It is shaded somewhat more toward the magenta than electric purple and it is displayed in the color box at right.

In the 1980s there was a Jimi Hendrix Museum in a Victorian house on the east side of Central Ave. one half block south of Haight Street in the Haight-Ashbury neighborhood of San Francisco which was painted this color.

Mulberry

The color mulberry is displayed at right. This color is a representation of the color of mulberry jam or pie. This was a Crayola crayon color from 1958 to 2003.

The first recorded use of Mulberry as a color name in English was in 1776.

Pansy purple

The pansy flower has varieties that exhibit three different colors: pansy (a deep shade of violet), pansy pink, and pansy purple.

The first recorded use of Pansy Purple as a color name in English was in 1814.

Tokyo purple

Tokyo purple is the color of the flag of the prefecture of Tokyo in the Kantō region of Japan. This shade of dark purple has been associated with Tokyo (formerly called Edo) since ancient times.

Purple in nature

Plants

Purple needlegrass is the state grass of California.

Animals

The purple frog is a new species of amphibian discovered in India in 2003.

Purple in human culture

Anti-Apartheid Movement

The Purple Rain Protest was a protest against apartheid that took place in Cape Town, South Africa on 2 September 1989, in which a police water cannon with purple dye sprayed thousands of demonstrators. This led to the slogan The purple shall govern.

Astronomy

One of the stars in the Pleiades, called Pleione, is sometimes called Purple Pleione because, being a fast spinning star, it has a purple hue caused by its blue-white color being obscured by a spinning ring of electrically excited red hydrogen gas.

Billiard games

Purple is the color of the ball in Snooker Plus with a 10-point value.

Calendars

Purple is associated with Saturday on the Thai solar calendar. Anyone may wear purple on Saturdays and anyone born on a Saturday may adopt purple as their color.

Comedy

The Purple Onion is a celebrated comedy club in the North Beach area of San Francisco, California.

Dance

The Purple Moon Dance Project is a dance group in San Francisco.

Fraternities and sororities

Sigma Lambda Beta recognizes royal purple and pure white as its official fraternal colors.
Sigma Lambda Gamma recognizes majestic purple and shocking pink as its official colors.
Omega Psi Phi recognizes royal purple and old gold as their official colors.
Delta Phi Epsilon recognizes Royal Purple and Pure Gold as its official colors.

Heraldry

Porpora, or purpure, was not one of the usual tinctures in European heraldry, being added at a late date to bring the number of tinctures plus metals to seven, so that they could be given planetary associations. The classic early example of purpure is in the coat of arms of the Kingdom of León: argent, a lion purpure, as early as 1245.

History

Byzantine empresses gave birth in the Purple Chamber of the palace of the Byzantine Emperors. Therefore, being named Porphyrogenitus ("born to the purple") marked a dynastic emperor as opposed to a general who won the throne by his effort.
In China, the Chinese name of the Forbidden City literally means "purple forbidden city" 紫禁城 with first character 紫 meaning purple (even though the Chinese Emperor himself wore yellow, which was considered in China to be the imperial color).

Holocaust

The purple triangle was a Nazi concentration camp badge used by the Nazis to identify several un-orthodox non-conformist religious groups known as Bibelforscher, mostly Jehovah’s Witnesses.

Literature

Alice Walker, author of The Color Purple, said, "Womanist is to feminist as purple is to lavender."
As a result of its association with royalty and luxury, the term 'purple' is often used to describe pretentious or overly embellished literature. For example, a paragraph containing an excessive number of long and unusual words is called a purple passage (see Purple prose).

Microbiology

In April 2007 it was suggested that early archaea may have used retinal, a purple pigment, instead of chlorophyll, to extract energy from the sun. If so, large areas of the ocean and shoreline would have been colored purple; this is called the Purple Earth theory.

Military

In the United States and United Kingdom militaries, purple refers to programs or assignments that are "joint", i. e., that are not confined to a single service such as the Army or Navy, but apply to the entire defense establishment. Assignment to one or more joint billets is required for promotion to flag rank (Rear Admiral and higher) in the U.S. Navy. Officers in joint billets are sometimes referred to as "wearing purple" (the phrase is purely metaphorical as there are no purple uniforms in the U.S. or UK armed forces).
During and before World War II, the Japanese used a code known as PURPLE or the Purple Code. The Allies' military successes in the Pacific theater depended on the fact that the Japanese did not know that Allied cryptographers had broken the code.
The Purple Heart is a United States military decoration awarded in the name of the President of the United States to those who have been wounded or killed while serving on or after 5 April 1917 with the U.S. military.

Music

Purple Rain, song by Prince
Start Wearing Purple is a song by gypsy punk band Gogol Bordello.
Deep Purple is a rock band.
"Deep Purple" (song) is also the name of a popular song that was the favorite song of Babe Ruth.
"Purple People Eater" was one of the biggest rock and roll hits of 1958.
"Purple Haze" is one of the most popular songs by Jimi Hendrix.
Purple is the favorite color of the pop celebrity Prince. His 1984 film and album Purple Rain is one of his best known works. The title track is Prince's signature song and is nearly always played in concert to this day. Prince encourages his fans to wear purple to his concerts.
Purple Ribbon Records is a hip-hop record label owned by rapper Big Boi of the rap duo Outkast. 2005 saw the release of the mixtape Got Purp? Vol 2 featuring the Purple Ribbon All-Stars and other artists on the label.
Purple is a 1994 album by the band Stone Temple Pilots.
Purple Music, Inc is a company in Switzerland that produces house music.
The New Riders of the Purple Sage is an American country rock band. The group emerged from the psychedelic rock scene in San Francisco in 1969, and its original lineup included members of the Grateful Dead.
Purple was Frank Zappa's favorite color.
The Purple Bottle is a song by noise pop band Animal Collective from their Feels album.
Purple Radio is an internet radio station for musically obsessed people to share their favorite music

Oenology

There is a winemaker in the Napa Valley of Northern California named Alex Cose who is with a company called the Purple Wine Co. that specializes in the production of wines for restaurants that is poured by the glass.

Parapsychology

People with purple auras are said to have a love of ritual and ceremony.

Politics

In the politics in the Netherlands, purple ("paars" in Dutch) means a government coalition of right-liberals and socialists (symbolized by blue and red, respectively), as opposed to the more common coalitions of the Christian-democratic center-party with one of the other two. From 1994 to 2002 there have been two purple cabinets.
In United States politics, a purple state is a state equally balanced between Republicans (normally symbolized by red) and Democrats (normally symbolized as blue)(See red states and blue states).
In British politics, purple is used to represent the United Kingdom Independence Party, a eurosceptic party wanting to pull Britain out of the European Union

Religious text

In the Byzantine Empire, Gospel manuscripts were written in gold lettering on parchment that was colored Tyrian purple.

Rhyme

Robert Burns rhymes purple with "curple" in his Epistle to Mrs. Scott. Burns is, as far as we can tell, the only writer to have used the word. A curple refers to 1) the small of the waist before the flare of the hips or 2) a derriere, rump or behind.
Curple is a word out of Scotland, which refers to the hindquarters of a horse. The current Shorter OED lists 'curple' dating from 1591.
In the song Grace Kelly by Mika the word purple is rhymed with "hurtful".
In his hit song "Dang Me," Roger Miller sings these lines:

Roses are red, violets are purple
Sugar is sweet and so is maple surple [sic]

Sexuality

Today the color purple is also known as a "pride" color among the gay community.

At the 24 June 2007 San Francisco Gay Pride Parade, Yahoo passed out 3 7/16" in diameter round plastic stickers with a picture of a gay man or woman imaged as one of the Yahoo Gay Pride avatars against an HTML/CSS Purple background that said Out, Proud, and Purple.
In the mid 1970s, there was a gay piano bar at 2223 Market St. between Noe and Castro in San Francisco called the Purple Pickle.

Science fiction

In the Star Trek universe, Klingons have purple blood.
In the Star Wars universe, Jedi Master Mace Windu has a purple lightsaber.
Trance Gemini from Gene Roddenberry's Andromeda was purple in the first few seasons.

Sports

The National Basketball Association's Los Angeles Lakers, Phoenix Suns and Sacramento Kings use purple as their primary color, though the Lakers formerly used the term "Forum Blue", in reference to their old arena The Forum. The National Hockey League's Los Angeles Kings use purple as one of their primary colors. In Major League Baseball, purple is one of the primary colors for the Colorado Rockies. In the National Football League, the Minnesota Vikings and Baltimore Ravens use purple as main colors. The Australian Football League's Fremantle Dockers use purple as one of their primary colors. In Association football (Soccer) Australian A-League Club Perth Glory use purple as one of their primary colors and Italian Serie A club Fiorentina use purple as their primary color.

Transpersonal psychology

In 1976, a chart by Timothy Leary and Robert Anton Wilson called The Periodic Table of Energy outlining the philosophy of Dr. Timothy Leary (The Eight Circuit Model of Consciousness)was given out by the Starflight Network, a group in Berkeley, California that was founded by Robert Anton Wilson to promulgate Dr. Timothy Leary's philosophy. The Eighth or Psycho-Atomic Circuit was represented on the chart by the color psychedelic purple.