A person uses lenses of +2.0 D power in his spectacles for the correction of his vision.
(a) Name the defect of vision the person is suffering from.
(b) List two causes of this defect.
(c) Determine the focal length of the lenses used in the spectacles.
Generated by claude-sonnet-4-6 · 2026-06-15 06:53 · grounding rag
Model Answer
(a) The person is suffering from Hypermetropia (far-sightedness). A positive (+) power lens is a convex lens, used to correct this defect.
(b) Two causes of Hypermetropia:
- The focal length of the eye lens is too long.
- The eyeball has become too small.
(c) Given: Power, P = +2.0 D
Using $P = \dfrac{1}{f}$:
$$f = \frac{1}{P} = \frac{1}{+2.0} = +0.5 \text{ m}$$
The focal length of the lens is +0.5 m.
Source: Chapter 10, Section 10.2 (Hypermetropia); Chapter 9, Section 9.3.8 (Power of a Lens)
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Explanation
- The positive power directly tells you the lens is convex, which is used for hypermetropia — this is the key clue for part (a).
- For causes, use the exact two points from the textbook: eyeball too small OR focal length too long. Examiners award marks for these specific phrases.
- For part (c), the formula $f = 1/P$ is straightforward. Always state the unit (metres). Showing the substitution step earns full method marks even if you make an arithmetic slip.