Did you ever look at the edge of a mirror, where the glass is cut at an angle, and see all the colors of the rainbow there? The glass was acting as a prism. What was taking place was the “dispersion” of light; what you saw was the “spectrum.” Now let’s see what all of this means.

When light passes from air into water, or from air into glass, it is changed in direction. This change is called “refraction,” and you’ve probably noticed it thousands of times! For example, you’ve seen a spoon slanting into a glass of water. At the surface, the spoon looks as if there’s a sharp bend in its handle. This is because the water has bent the beam of light slightly.