Answer
A converging lens is thicker in the middle than at the edges, and brings parallel rays to a focus at a point.
A diverging lens is thicker at the edges than in the middle, and makes parallel rays diverge. The diverging rays can be extrapolated back to a focal point on the back side of the lens.
Work Step by Step
This is discussed on pages 533-534, and shown in Figure 28.43.
