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The index of refraction of a given material depends on the wavelength (or frequency) of the incoming light. Hence, the speed of light in that material also depends on the wavelength or frequency of light. Restart.
When the index of refraction of a material is given, therefore, it is really true for only one particular wavelength or color of light. This slight variation in the index of refraction leads to what are called chromatic aberrations in lenses (where the focal point is different for different colors). It is what allows for the separation of white light into colors using a prism (or drops of water). This phenomenon is called dispersion. When the speed of a wave in a particular medium is a function of frequency, the medium is dispersive. Note that for this Illustration we consider the dispersive qualities of glass (1.6 < n < 1.68), but air itself is also dispersive (1.45 < n < 1.47).
Change the wavelength of light (in air) and therefore change the color of the light entering the prism. Notice the angle at which the different colors exit the prism and the different index of refraction associated with each color.
When white light enters the prism, what happens? This is a very nice example of dispersion in glass. You see a rainbow of colors, both inside and outside of the prism, because each light ray refracts differently depending on its wavelength (or frequency). A raindrop can also refract sunlight. The result of the dispersion of light in water droplets during a passing rainstorm is often a rainbow.
Illustration authored by Morten Brydensholt and Anne J. Cox.
Script authored by Morten Brydensholt.
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© 2004 by Prentice-Hall, Inc. A Pearson Company