Dispersion in
Wavelength

In the preceding section we showed that light is refracted at the boundary between two media differing in density. Let us now examine more carefully the factors upon which this refraction depends.

Factors Governing Refraction

The amount of refraction of light at a boundary between two media depends on three things:
  1. The nature of the media (embodied in a characteristic quantity called the index of refraction for a medium).

  2. The angle of indidence for the light ray on the boundary.

  3. The wavelength of light.
The dependence of refraction on the wavelength of light is called dispersion. This dependence has both positive and negative implications for astronomy. On the positive side, it is the basis for the prism and its ability to separate light according to wavelength; on the negative side, it is the source of chromatic aberration in optical devices (the failure of different wavelengths to focus at the same point).

Dispersion and The Prism

Dispersion is the basis for the prism and its ability to spatially separate light according to wavelength, as illustrated in the following animation.


Light separated into its freqency (and therefore energy or wavelength) components is called a spectrum of light. Here are Java applets illustrating the dispersion of visible light by a triangular prism and by a rectangular glass slab.

Spectrographs

A spectrograph is a refined instrument that produces a spectrum. Although a prism can disperse light according to color, in modern spectrographs it is more common to accomplish the same task by using a diffraction grating. The diffraction grating works on a completely different principle (diffraction rather than refraction) but it also can separate light spatially according to wavelength. We will see in subsequent sections that the spectrograph is a central tool of modern astronomy.


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