(a) Define the power of accommodation of the human eye. How do the ciliary muscles bring about this accommodation when viewing a nearby object versus a distant object?
(b) A myopic person's far point is 2 m. Calculate the power of the lens he needs to see distant objects clearly.
(c) With increasing age, a person finds that the range of clear vision becomes very restricted — she cannot see near objects or far objects without glasses. Name the defect and describe the type of lens used to correct it.
Generated by claude-sonnet-4-6 · 2026-06-26 01:09 · grounding rag
Model Answer
(a) The ability of the eye lens to adjust its focal length to see objects at different distances is called the power of accommodation.
- Distant object: Ciliary muscles relax → lens becomes thin → focal length increases → distant object seen clearly.
- Nearby object: Ciliary muscles contract → lens becomes thicker → focal length decreases → nearby object seen clearly.
(b) Far point of myopic person = 2 m. The concave lens must form the image of a distant object (at infinity) at the far point (2 m).
$$P = \frac{1}{f} = \frac{1}{-2\text{ m}} = -0.5 \text{ D}$$
The person needs a concave lens of power −0.5 D.
(c) The defect is Presbyopia. It occurs due to gradual weakening of ciliary muscles and reduced flexibility of the eye lens with age. Since the person cannot see both near and distant objects, bi-focal lenses are used — the upper part has a concave lens (for distant vision) and the lower part has a convex lens (for near vision).
Source: Chapter 10, Sections 10.1.1 and 10.2
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Explanation
- (a) Examiners expect the definition plus both cases (near and far) with the muscle action and lens shape change clearly stated — don't skip either case.
- (b) Use $P = 1/f$; since the far point is 2 m, $f = -2$ m (concave lens). Show the formula and substitution for full marks.
- (c) The keyword "restricted range — neither near nor far" is the clue for presbyopia (not just myopia or hypermetropia). Always name the specific lens for each part of the bi-focal — examiners look for this detail.