A VOYAGE IN SPACE
for some distance, and then when it hits the surface of the ground it "breaks back" owing to the spin the bowler has given it? Well, a beam of light behaves in much the same way when it is refracted; after going straight for some distance, it strikes a surface and turns off at an angle.
In Fig. 24 the beam of light AB goes along straight from A to B, and then strikes SF, the surface of glass or water; thereupon instead of continuing straight along BC it is "broken back," or is refracted along BD. Now when we use a lens, we have two surfaces to deal with, one where the light goes in and one where it comes out, and both of these are curved, which is a new complication. To keep matters as simple as possible I will first use a prism; that is, a piece of glass with two flat surfaces. When the ray AB strikes the first surface SF (Fig. 25), it is broken back as we have said along BD; and when it comes to the second surface to get out of the glass again it is broken back again along DE. You may think that I have drawn DE the wrong way and that it should have been in some such direction as DX; but