Final answer:
The lens thickens when looking at a close object as the ciliary muscle fibers contract, and it becomes thinner when looking at a distant object as the ciliary muscle fibers relax.
Step-by-step explanation:
When looking at a close object, the lens thickens as the ciliary muscle fibers contract; when looking at a distant object, the lens becomes thinner as the ciliary muscle fibers relax.
Figure 26.4 illustrates the accommodation of the eye, where the lens is made more converging for close vision by the action of the ciliary muscle. This allows light rays from nearby objects, which can diverge, to enter the eye and form a clear image on the retina.
On the other hand, when looking at distant objects, the lens becomes thinner and less powerful. This difference in lens shape and power is due to the contraction and relaxation of the ciliary muscle fibers in response to changes in object distance.