Dispersion and Spectra
Unlike a plate glass, with its parallel sides, a prism, since its sides are not parallel, changes the direction of light when it passes through it.
And a prism does even more than change the direction of the light. Light of different wavelengths or colors have different speeds -- or different indices of refraction. This means the bending of light by a prism is somewhat different for different wavelengths or colors.
The bending of light by a prism is somewhat different for different wavelengths or colors.
This means white light is spread out into a spectrum of colors as it passes through a prism. This also explains the "fire" or "sparkle" in gemstones such as diamonds or cubic zirconia.
(C) 2003, Doug Davis; all rights reserved