Red

Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondary color (made from magenta and yellow) in the CMYK color model, and is the complementary color of cyan. Reds range from the brilliant yellow-tinged scarlet and vermillion to bluish-red crimson, and vary in shade from the pale red pink to the dark red burgundy.

Red pigment made from ochre was one of the first colors used in prehistoric art. The Ancient Egyptians and Mayans colored their faces red in ceremonies; Roman generals had their bodies colored red to celebrate victories. It was also an important color in China, where it was used to color early pottery and later the gates and walls of palaces. In the Renaissance, the brilliant red costumes for the nobility and wealthy were dyed with kermes and cochineal. The 19th century brought the introduction of the first synthetic red dyes, which replaced the traditional dyes. Red became a symbolic color of communism and socialism; Soviet Russia adopted a red flag following the Bolshevik Revolution in 1917, until the breakup of the Soviet Union in 1991. Communist China adopted the red flag following the Chinese Revolution of 1949. It was adopted by North Vietnam in 1954, and by all of Vietnam in 1975.

Since red is the color of blood, it has historically been associated with sacrifice, danger, and courage. Modern surveys in Europe and the United States show red is also the color most commonly associated with heat, activity, passion, sexuality, anger, love, and joy. In China, India, and many other Asian countries it is the color symbolizing happiness and good fortune.

Shades and variations

Varieties of the color red may differ in hue, chroma (also called saturation, intensity, or colorfulness), or lightness (or value, tone, or brightness), or in two or three of these qualities. Variations in value are also called tints and shades, a tint being a red or other hue mixed with white, a shade being mixed with black. Four examples are shown below.

File:Cardinal.jpg, The cardinal takes its name from the color worn by Catholic cardinals.
File:Cherry blossoms in the Tsutsujigaoka Park.jpg, Pink is a pale shade of red. Cherry blossoms in the Tsutsujigaoka Park, Sendai, Miyagi, Japan.
File:Red tikka powder.jpg, Vermilion is similar to scarlet, but slightly more orange. This is sindoor, a red cosmetic powder used in India; some Hindu women put a stripe of sindoor in their hair to show they are married.
File:Ruby gem.JPG, Ruby is the color of a cut and polished ruby gemstone.

In science and nature

Seeing red

The human eye sees red when it looks at light with a wavelength between approximately 625 and 740 nanometers. 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. 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.

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. (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. This may have driven further adaptations by species taking advantage of this new ability, such as the emergence of red faces.

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.

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 ed 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.

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 principle is used to generate colors on such as computer monitors and televisions. 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.

(See Web colors and RGB color model)

File:Boutet 1708 color circles.jpg, In a traditional color wheel from 1708, red, yellow and blue are primary colors. Red and yellow make orange; red and blue make violet.
File:RGB illumination.jpg, 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.
File:RGB pixels.jpg, 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.
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.

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, 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. 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.

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.

Astronomy

* Mars is called the Red Planet because of the reddish color imparted to its surface by the abundant iron oxide present there.
* 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.
* 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.
* Interstellar reddening is caused by the extinction of radiation by dust and gas

Mars atmosphere 2.jpg, Mars appears to be red because of iron oxide on its surface.
Mira 1997.jpg, Mira, a red giant
File:RedDwarfNASA.jpg, Artist's impression of a red dwarf, a small, relatively cool star that appears red due to its temperature

Pigments and dyes

Roussillon sentier des ocres2.JPG, Red ochre