Red is the color at the end of the spectrum of visible light next to orange and opposite violet. It is evoked by light with a predominant wavelength of roughly 620–740 nm. Red is one of the additive primary colors of light, along with green and blue, which in RGB color systems are combined to create all the colors on a computer monitor or television screen. It is also one of the subtractive primary colors, along with yellow and blue, of the RYB color space and traditional color wheel used by painters and artists. Reds can vary in shade from very light pink to very dark maroon or burgundy; and in hue from the bright orange-red scarlet or vermilion to the bluish-red crimson. Red was widely used in prehistoric cave art, made with red hematite or iron oxide, or red ochre. Early civilizations in China, the Middle East and Europe made red dyes from the madder plant or from the a tiny insect called the kermes vermilio, Civilizations in the Americas made brilliant reds from another scale insect, the cochineal, Ancient Chinese artists during the Han Dynasty used dye from the madder plant to color silk and pigments from lead oxide to color pottery. In ancient Rome red symbolized blood and courage; Roman soldiers wore red tunics, and generals wore a scarlet cloak, and had their bodies painted completely in red during ceremonial triumphs. It also sometimes represented sin; the Book of Revelation described the ‘’scarlet whore of Babylon.” In the Middle Ages, red was the color of majesty; it was the color of the banners of the Byzantine Empire, and the Emperor Charlemagne painted his palace red and wore red shoes as symbol of his authority. In Christianity it represented the blood of Christ and the Christian martyrs; in 1295 it became the color worn by Cardinals of the Roman Catholic Church. Merchants and ordinary people wore clothing dyed with madder at celebrations. Nobles and merchants wore the more intense red made with Kermes. In the 16th century, Spanish merchants began to import cochineal into Europe, which made an even more brilliant red; it became the fashion color of the aristocracy. Red also played an important role in the culture of Imperial China. In Chinese philosophy, red represented fire, one of the five elements, During the Zhou, Han, Jin, Song and Ming Dynasties, red was considered a noble color, and it was featured in all court ceremonies, from coronations to sacrificial offerings, and weddings. The gates of imperial palaces were usually painted red. Chinese artisans used the mineral cinnebar to produce the famous vermilion or “Chinese red” color of Chinese lacquerware. During the French Revolution, some of the revolutionaries began to wear a red Phrygian cap, or liberty cap, modeled after the caps worn by freed slaves in Ancient Rome, and to carry red flags. In the mid-19th century, red became the color of a new political and social movement, socialism. It became the color of the worker’s movement, of the French Revolution of 1848, of the Paris Commune in 1870, and of socialist parties across Europe. In 1826 the organic compound alizarin, the coloring ingredient of madder, was discovered and in 1848 it was synthesized; it was cheaper and made a more lasting color, and quickly replaced natural vegetable dyes. 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. 19th century artists adopted the color alizarin crimson. In the 19th century artists also began to use red to create specific emotions, not just to imitate nature; Vincent Van Gogh wrote in 1888, “I sought to express with red and green the terrible human passions.” 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 nature, the red color of blood comes from hemoglobin, the iron-containing protein found in the red blood cells of all vertebrates. Hemoglobin transports oxygen from the lungs to the rest of the body, where it is released to provide energy to power the functions of the body. The red color of the Grand Canyon and other geological features is caused by hematite or red ochre, both forms of iron oxide. It also causes the red color of the planet Mars. The red sky at sunset and sunrise is caused by an optical effect known as Rayleigh scattering, which, when the sun is low or below the horizon, increases the red-wavelength light that reaches the eye. The color of autumn leaves is caused by pigments called Anthocyanins, which are produced towards the end of summer, when the green chlorophyll is no longer produced. One to two percent of the human population has red hair; the color is produced 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. Since red is the color of blood, it has historically been associated with sacrifice, danger and courage. Modern surveys in the United States and Europe show red is also the color most commonly associated with heat, activity, passion, sexuality, anger, love and joy. In China and many other Asian countries it is the color of happiness.