When white light passes through a prism its direction is changed and it is spread out into various colors. This spreading of the light into various colors is called a spectrum. This spreading of colors in the white light is because the index of refraction varies with the wavelength (or color) of the light. The variation of index of refraction with wavelength is called dispersion.

Rainbows are caused by the dispersion of light as it is refracted and reflected by tiny droplets of water. The angle between the center of a rainbow and the rainbow itself is about 45^o. The colors of the spectrum are present in a rainbow with violet and blue on the inside and red on the outside. A second rainbow is formed by light that is reflected twice inside the rain droplets. The colors on a second rainbow are reversed.

Rods and cones on the retina of our eyes allow us to see. The cones are color-sensitive and require higher levels of illumination. The rods are far less color-sensitive and come into play when the light is dim.

Color addition applies for spotlights or color television. The additive primary colors are red, green, and blue.

Color subtraction applies for paints or color printing. The subtractive primary colors are cyan, yellow, and magenta.