Bifocals have both a diverging and converging lens. The aging process causes one's visual far point to become too close and the near point to become too far away. Single-vision lenses are insufficient to correct both problems.
Work Step by Step
As people age, the lenses in their eyes become less flexible. Their eyes cannot accommodate as well (discussed on page 719), meaning it’s harder for the lens to change shape and bring images to a focus. The net result is that their far point is too close to their eyes, and their near point is too distant. Bifocals can correct both of these issues – one looks up through a diverging lens at distant objects, and down through a converging lens at nearby objects.