Red

Red is a color associated with the long-wavelength part of visible light. In the electromagnetic spectrum, red light has wavelengths of roughly 620 to 750 nanometers, although the exact limits used to define visible colors are not fixed. Among the main spectral colors, red is at the end with the longest wavelengths and the lowest frequencies. Our perception of red begins when light stimulates the cone cells in the retina in a particular pattern.

The long-wavelength-sensitive cones respond especially strongly to reddish light, while the brain compares their signals with those from the other cone types to determine the color we see. A red object does not necessarily produce red light itself. Under ordinary illumination, it usually appears red because its surface reflects more red wavelengths toward our eyes and absorbs or scatters other parts of the incoming light. The appearance can change with the type of illumination and with nearby colors.

Red occurs widely in nature. Many plants contain pigments that give flowers, fruits, and leaves red shades. Anthocyanins can produce red, purple, or blue colors depending on their chemical environment, while carotenoid pigments contribute red or orange colors in some plants. Animals may also appear red because of pigments in their tissues, blood, feathers, scales, or skin, although the underlying mechanisms vary greatly between species.

Humans use red in art, clothing, signs, maps, screens, and many other forms of communication. On digital displays, red is one of the three primary light channels in the RGB system, where red, green, and blue light are combined at different intensities to create a wide range of colors. Mixing physical pigments works differently because pigments selectively absorb light. Red therefore provides a useful example of how color depends on light, materials, illumination, and the way the visual system interprets incoming signals.