Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of roughly 625–740 nm. Longer wavelengths than this are called infrared (below red), and cannot be seen by the naked human eye. Red is used as one of the additive primary colors of light, complementary to cyan, in RGB color systems. Red is also one of the subtractive primary colors of RYB color space but not CMYK color space.

In human color psychology, red is associated with heat, energy, and blood, and emotions that "stir the blood", including anger, passion, and love.

Etymology and definitions

The word red comes from the Old English rēad. Further back, the word can be traced to the Proto-Germanic rauthaz and the Proto-Indo European root reudh-. This is the only color word which has been traced to an Indo-European root. In Sanskrit, the word rudra means red. In the English language, the word red is associated with the color of blood, certain flowers (e.g. roses), and ripe fruits (e.g. apples, cherries). Fire is also strongly connected, as is the sun and the sky at sunset. Healthy light-skinned people are sometimes said to have a "ruddy" complexion (as opposed to appearing pale). After the rise of socialism in the mid-19th century, red was to describe revolutionary movements. The word is also obviously associated with anything of the color occupying the lower end of the visible light spectrum, such as red hair or red soil.

In science

Colorimetry, color science, vision, and photography

Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of approximately 630–700 nm. Longer wavelengths than this are called infrared, or below red and cannot be seen by human eyes. Red's wavelength has been an important factor in laser technologies as 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 blue lasers. Red light is also used to preserve night vision in low-light or night-time situations, as the rod cells in the human eye aren't sensitive to red. Red is used as one of the additive primary colors of light, complementary to cyan, in RGB color systems. Red is also one of the subtractive primary colors of RYB color space but not CMYK color space.

One common use of red as an additive primary color is in the RGB color model. Because "red" is not by itself standardized, color mixtures based on red are not exact specifications of color either. In order to produce exact colors the color red needs to be defined in terms of an absolute color space such as sRGB. As used in computer monitors and television screens, red is very variable, but some systems may apply color correction (so that a standardized "red" is produced that is not in fact full intensity of only the red colorant).