White is a tone, the perception which is evoked by light that stimulates all three types of color sensitive cone cells in the human eye in near equal amount and with high brightness compared to the surroundings.

Since the impression of white is obtained by three summations of light intensity across the visible spectrum, the number of combinations of light wavelengths that produce the sensation of white is practically infinite. There are a number of different white light sources such as the midday Sun, incandescent lamps, fluorescent lamps and white LEDs. The impression of white light can also be created by mixing appropriate intensities of the primary colors of light ? red, green and blue (RGB) ? a process called additive mixing, as seen in many display technologies.

White light reflected off objects can be seen when no part of the light spectrum is reflected significantly more than others and the reflecting material has a degree of diffusion. This is seen when transparent fibers, particles, or droplets are in a transparent matrix of a substantially different refractive index. Examples include classic "white" substances such as sugar, foam, pure sand or snow, cotton, clouds, and milk. Crystal boundaries and imperfections can also make otherwise transparent materials white, as in the milky quartz or the microcrystalline structure of a seashell. This is also true for artificial paints and pigments, where white results when finely divided transparent material of a high refractive index is suspended in a contrasting binder. Typically paints contain calcium carbonate and/or synthetic rutile with no other pigments if a white color is desired.

The visible spectrum (or sometimes called the optical spectrum) is the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths in air from about 380 to 750 nm. The corresponding wavelengths in water and other media are reduced by a factor equal to the refractive index. In terms of frequency, this corresponds to a band in the vicinity of 400?790 terahertz. A light-adapted eye generally has its maximum sensitivity at around 555 nm (540 THz), in the green region of the optical spectrum (see: luminosity function). The spectrum does not, however, contain all the colors that the human eyes and brain can distinguish. Unsaturated colors such as pink, and purple colors such as magenta are absent, for example, because they can only be made by a mix of multiple wavelengths.

Wavelengths visible to the eye also pass through the "optical window", the region of the electromagnetic spectrum which passes largely unattenuated through the Earth's atmosphere (although blue light is scattered more than red light, which is the reason the sky appears blue). The response of the human eye is defined by subjective testing (see CIE), but the atmospheric windows are defined by physical measurement. The "visible window" is so called because it overlaps the human visible response spectrum; the near infrared (NIR) windows lie just out of human response window, and the Medium Wavelength IR (MWIR) and Long Wavelength or Far Infrared (LWIR or FIR) are far beyond the human response region.

The eyes of many species perceive wavelengths different from the spectrum visible to the human eye. For example, many insects, such as bees, can see light in the ultraviolet, which is useful for finding nectar in flowers. For this reason, plant species whose life cycles are linked to insect pollination may owe their reproductive success to their appearance in ultraviolet light, rather than how colorful they appear to our eyes. Birds are also said to be able to see into the ultraviolet (300-400 nm) and oddly enough, the sex-dependent markings on some bird plumage is only visible in the ultraviolet range.