Answer
Light "bends" in the optical fiber because it is totally internally reflected over and over.
Work Step by Step
The light bounces off the inner walls without losing intensity, and follows the fiber's twists and turns (as long as any kinks in the fiber are not too severe). The textbook likens the situation to a bullet ricocheting down a steel pipe.
This is discussed on page 533, and shown in Figure 28.42.
