Q1. [2]
A person suffering from presbyopia needs bifocal lens. If he needs two lens of power −4·0 dioptre and +2·0 dioptre, which one of these two lenses is for the correction of distant vision and what is its focal length ?
Previously asked in CBSE board exam
2024 31/4/1 Q25
Generated by claude-sonnet-4-6 · 2026-06-15 06:52 · grounding rag
Model Answer
The lens of power −4.0 D is for the correction of distant vision (myopia), as it is a concave lens (negative power).
Its focal length:
$$f = \frac{1}{P} = \frac{1}{-4.0} = -0.25 \text{ m}$$
The focal length of the lens for distant vision is −0.25 m.
Explanation
- In bifocal lenses for presbyopia, the concave (negative power) lens corrects distant vision (myopia) and occupies the upper portion; the convex (positive power) lens corrects near vision (hypermetropia) and occupies the lower portion.
- Use $f = \frac{1}{P}$ with $P$ in dioptres to get $f$ in metres. Don't forget the negative sign — examiners check it.
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