Abney's Law

Abney's law, named for English chemist and physicist Sir William de Wiveleslie Abney (1843-1920), states the principle that explains that the "total luminance" or full spectrum of light (white light) is composed of several wavelengths and is equal to the sum of luminances of its monochromatic (single color frequencies) components. This is why sunlight can be fractionated by rain to create a rainbow or through the use of a prism to create a light spectrum.