An elderly person uses bi-focal lenses. (i) What are bi-focal lenses? (ii) Describe the function of each portion of a bi-focal lens. (iii) Which defect(s) of vision make bi-focal lenses necessary?
Generated by claude-sonnet-4-6 · 2026-06-26 01:06 · grounding rag
Model Answer
(i) Bifocal lenses are special lenses that have two different portions — an upper portion and a lower portion — each with a different focal length.
(ii) Function of each portion:
- Upper portion — made of a concave lens; used for distant vision (corrects myopia).
- Lower portion — made of a convex lens; used for near vision/reading (corrects hypermetropia).
(iii) Defects corrected:
Bifocal lenses are necessary when a person suffers from both myopia (short-sightedness) and hypermetropia (long-sightedness) simultaneously. This condition commonly occurs in old age and is called presbyopia.
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Explanation
- Examiners expect all three parts answered clearly and separately.
- Key terms to use: concave (upper/distant), convex (lower/near), myopia, hypermetropia, presbyopia.
- The word "presbyopia" scores marks in part (iii) — do not omit it.
- Keep each part concise; this is a 3-mark question so ~1 mark per part.