CBSE Class 10 Science 2019 · Paper 31/3/3 · Question 3
List two causes of presbyopia. Draw labelled diagram of a lens used for the correction of this defect of vision.
Answer
Weak ciliary muscles and a less flexible eye lens cause presbyopia; a bifocal lens (upper concave, lower convex) corrects it.
Causes of presbyopia: (i) gradual weakening of the ciliary muscles; (ii) diminishing flexibility of the eye lens (both with age).
Lens used for correction: a bifocal lens. Its upper portion is a concave lens, for distant vision, and its lower portion is a convex lens, for near vision (reading); the labelled diagram is at step 4. When only near vision is affected, a convex lens of suitable power is used.
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From the board paper
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