Red

Red (also called casaan or gaduud) is one of the primary colors visible to the human eye. It is produced by light with wavelengths in the long end of the visible spectrum, ranging from about 620–750 nanometres (a nanometre is one billionth of a metre). Red light is followed by the infrared portion of the spectrum, which the eye cannot see.

Red is counted as one of the three primary colors in the RGB (red, green, blue) additive light system used in television and computer screens. In subtractive color printing systems, red is produced by mixing magenta and yellow.

Human vision

The retina of the eye contains cells called cones, of which there are three types. The cone types sensitive to longer wavelengths respond strongly to red light. People with color blindness, a condition known as protanopia, may have difficulty distinguishing red from green. Red is also perceived quickly by the brain, which explains its use in warning signs and traffic signals.

Natural sources

Natural red comes from various sources. Animal blood gets its red color from haemoglobin, which contains iron. Tomatoes and certain vegetables contain lycopene, while the red of carrots comes from other carotenoids. In soil, iron oxide gives earth and rock their reddish colour. When the sun rises or sets, the sky turns red as light with shorter wavelengths is scattered in the atmosphere while longer wavelengths pass through.

Social use and symbolism

Red is used in the flags of many countries around the world, often symbolizing conflict or bloodshed. In some cultures it represents joy and good fortune, such as in Chinese weddings. Health organizations such as the Red Crescent and Red Cross use red symbols on a white background. In industry, red is used to mark fire-fighting equipment and danger warnings.