Color is determined by the wavelength of its light (the distance between one crest of the light wave and the next). Those colors that blend to form "white light" are from shortest wave length to longest: red, orange, yellow, green, blue, indigo, and violet. All these monochromatic colors, except indigo, occupy large areas of the spectrum (entire range of wavelengths produced when a beam of electromagnetic radiation is broken up). These colors can be seen when a light beam is refracted through a prism. Some consider the primary colors to be six monochromatic colors that occupy large areas of the spectrum: red, orange, yellow, green, blue, and violet. Many physicists recognize three primary colors: red, yellow, and blue; or red, green, and blue; or red, green, and violet. All other colors can be made from these by adding two primary colors in various proportions. Within the spectrum, scientists have discovered 55 distinct hues. Infrared and infra-violet rays at each end of the spectrum are invisible to the human eye. (The Handy Science Answer Book, compiled by the Science and Technology department of the Carnegie Library of Pittsburgh)