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.
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
triangular prism and by a
rectangular glass slab.
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
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.