Red

Red is a color found at the long wavelength end of the visible spectrum. It sits next to orange and is opposite to violet. The dominant wavelength for red is approximately 625 to 750 nanometers.

In the RGB color model, red is a primary color. In the CMYK color model, red is a secondary color created by mixing magenta and yellow. It is also the complementary color of cyan. Reds vary greatly in shade and hue. They range from bright, yellow-tinged scarlet and vermillion to bluish-red crimson. They also vary from pale pink to dark burgundy.

Human vision of red involves specific cells in the retina called cones. The eye sees red when light stimulates the L (long-wavelength) cone cells. This occurs without stimulating the S (short) or M (medium) cone cells. Light just beyond this range is called infrared. Humans cannot see infrared, but we can sense it as heat. Many mammals, such as dogs and cattle, possess dichromacy. This means they can see blues and yellows but cannot distinguish red from green. Bulls are a famous example of this. They do not charge a matador's cape because of the red color. Instead, they react to the motion of the fabric.

Humanity has used red for thousands of years. Prehistoric artists used red pigment made from ochre. Ancient Egyptians and Mayans used red for ceremonies. Roman generals colored their bodies red to celebrate victories. During the Renaissance, the wealthy wore red costumes dyed with kermes and cochineal. Cochineal is a dye made by crushing tiny female insects from Mexico and Central America. In the 19th century, the first synthetic red dyes were introduced. These new dyes eventually replaced traditional pigments. Red has also carried heavy political symbolism. It was the color of communism and socialism. The Soviet Union used a red flag following the 1917 Bolshevik Revolution. China and North Vietnam also adopted red flags after their respective revolutions.

Red also appears in the sky through a process called Rayleigh scattering. This happens during sunrises and sunsets. As white sunlight travels through the atmosphere, air molecules and particles scatter the light. Shorter wavelengths, like blue and green, scatter more strongly. These colors are removed from the beam of light. At sunset, the path through the atmosphere is longest. This removes the blue and green components almost completely. Only the longer orange and red wavelengths reach the eye.

In astronomy, red is a vital indicator of physical properties. Mars is called the Red Planet due to iron oxide on its surface. Stars also show red characteristics. Red giants are stars that have exhausted their core hydrogen. They switch to thermonuclear fusion in a shell surrounding the core. These stars can have radii hundreds of times larger than the Sun. Red supergiants, like Betelgeuse, are even larger. They can have radii 200 to 1700 times greater than the Sun. Red dwarfs are another type of star. They are small, cool, and very common in our galaxy.

Technology also relies on the specific properties of red light. The ruby laser was invented in 1960. In 1962, the red helium-neon laser was developed. These were used in holography and education. Modern DVD players use red laser diode technology at 660 nm. Red lasers were also used in early compact disc technology. However, they are being replaced by blue lasers. This is because red's longer wavelength requires more space on a disc.

Nature produces red through various chemical processes. In autumn, leaves turn red due to pigments called anthocyanins. These are not present during the whole growing season. They develop in late summer as phosphate moves out of the leaf. This change in chemistry causes sugars to break down into anthocyanins. Bright light and cool nights help create brilliant colors. Anthocyanins also color fruits like cherries, raspberries, and apples. In food, synthetic dyes like Allura Red AC are common. This dye is often made from petroleum. Some countries in Europe do not recommend it for children. Because of health concerns, some companies use natural carmine instead.