DISPERSION

The index of refraction for a particular substance depends on the wavelength of the light passing through it. Glass slows down light the most at the shortest wavelengths (blue and violet) and the least at the longest wavelengths (red and orange). This variation of the refractive index with wavelength is known as dispersion. It is the principle by which a prism works (Fig. 1). The more the light is slowed down by the glass, the more its path is deflected when it passes through the prism. This is why prisms cast rainbows when white light is shone through them.
Dispersion is important in optics for two reasons. First, a prism can be used to make a spectrometer, which is a device for examining the intensity of visible light at specific wavelengths. (Fine gratings are also used for this.) Second, dispersion degrades the quality of white-light images viewed through simple lenses.

Fig. 1. Dispersion is responsible for the fact that a glass prism “splits” white light into its constituent colors.