As the eye ages, its power of accommodation fades — many people then need bifocal lenses.
A bifocal lens: the upper half corrects distance vision, the lower half corrects near vision.
Presbyopia isn't caused by the shape of the eyeball — it's caused by time. As people age, the ciliary muscles weaken and the eye lens itself loses flexibility, so the power of accommodation gradually decreases.
The near point slowly recedes away from the normal 25 cm, making it harder to read or focus on close objects comfortably — even in someone who never had myopia or hypermetropia before.
Some people end up needing correction for both near and far vision at once. This is solved with bifocal lenses: the upper part is a concave lens for distant vision, and the lower part is a convex lens for near vision — one pair of glasses, two jobs.
Key exam points
Watch it explained
Human Eye and Colourful World — Defects of the Eye: Presbyopia, Causes and Correction · CBSE Class 10 Science