# Dispersion

In Section C we discussed the idea that the speed of light in each substance is related to its index of refraction n, and the index of refraction governs the bending of light as it crosses a boundary. Now, although this is true, it is not the whole truth.

A more complete version of the truth is that the index of refraction depends slightly on the frequency of the light. (Sometimes not so slightly, but that is another story.) The chart shows the index of refraction for glass. You can see that n is approximately 1.5, but not exactly. So blue light bends a little more when going from air to glass than red light. This is the principle behind the prism (Figure 13-23). The phenomenon is called dispersion. There are not many simple calculations we can do at this point, but you should study the diagram of the prism until it makes sense to you.

 f color n 4.6 X 1014 Hz red 1.4566 4.9 X 1014 Hz orange 1.4578 5.1 X 1014 Hz yellow 1.4584 5.8 X 1014 Hz green 1.4614 6.5 X 1014 Hz blue 1.4649 7.5 X 1014 Hz indigo 1.4702

Figure 13-23