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# Dispersion

"To err is human but to really foul things up requires a computer"
Anonymous
(Farmers Almanac 1978)

- Without explicitly making the statement we have assumed that the refractive index of a medium is independent of the frequency of the light transmitted. That is we have assumed for example that red light and blue light have the same speed when passing through glass (or any other medium). This is not exactly true, refractive index is frequency dependent such that for glass, nblue > nred (blue light is bent more than red light when passing from air to glass) and the speed of blue light in glass is slightly smaller than the speed of red light.
- This fact may be demonstrated by the production of a spectrum of colours when white light is incident on a glass prism. [White light is comprised of all the wavelengths of visible light] Each constituent of the white light has a slightly different refractive index and is deflected (according to Snell's law) through a slightly different angle.
- This phenomenon is known as DISPERSION. White light is dispersed into its constituent colours.
- A rainbow is created by a combination of dispersion and total internal reflection within water droplets in the atmosphere.
- An object appears a particular colour under white light illumination because it reflects that colour strongly.
  For example a red object reflects most of the red wavelength incident and absorbs most of the other wavelengths. If such an object is illuminated with blue light it will appear black, the blue light is absorbed, but there is no red to be reflected.

"There is no Dark Side of the Moon really. Matter of fact it's all dark"
Pink Floyd

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu