Light, also known as visible radiation, is electromagnetic radiation within the wavelength band that can be perceived by the human eye. This range is from about 380-780 nanometers (or nm; 1 nm = 10^-9 m). Wavelengths shorter than the lower end of the visible range are known as ultraviolet, and longer ones are infrared.

The electromagnetic spectrum of visible radiation is commonly partitioned into a number of sub-ranges, which correspond to the "colors of the rainbow": red, orange, yellow, green, blue, and violet. These colors have the following wavelength ranges: red, 740-620 nm; orange, 620-585 nm; yellow, 585-575 nm; green, 575-500 nm; blue, 490-445 nm; violet, 445-390 nm.

Sometimes, the word "light" is also used in reference to the entire electromagnetic spectrum, or to components such as gamma, X-ray, ultraviolet, infrared, microwave, or radio. In the physical sense, however, this is an inappropriate use of the word light, which should be restricted to wavelengths in the range of 380 to 780 nm.