A rainbow has seven colors because water droplets in the atmosphere break sunlight into seven colors. A prism similarly divides light into seven colors.

When light leaves one medium and enters another, the light changes its propagation direction and bends. This is called refraction. However, because of differences of refractive index, this refraction angle varies for each color-or according to the wavelength of the light. This change of the angle of refraction, or refractive index, in accordance with the wavelength of light is called dispersion. In conventional media, the shorter the wavelength (or the bluer the light), the larger the refractive index.