When you shift your gaze from a distant tree to a book held 25 cm away, describe what happens inside your eye to keep the image focused sharply on the retina. In your answer, name the structure responsible and explain the physical change it undergoes.
Generated by claude-sonnet-4-6 · 2026-06-26 01:07 · grounding rag
Model Answer
When we shift our gaze from a distant tree to a book held 25 cm away, the ciliary muscles (which control the eye lens) contract. This increases the curvature of the crystalline (eye) lens, making it thicker. As a result, the focal length of the eye lens decreases, converging the light from the nearby book sharply onto the retina. This ability of the eye lens to adjust its focal length is called accommodation.
(For a distant object, the ciliary muscles are relaxed, the lens becomes thin, and focal length increases.)
Source: Chapter 10, Section 10.1.1 — Power of Accommodation
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Explanation
- 3 marks are typically split as: (1) naming the structure — ciliary muscles / eye lens; (1) describing the physical change — muscles contract → lens becomes thicker / curvature increases; (1) effect — focal length decreases → image focused on retina.
- Always use the term accommodation — examiners expect it.
- Mention both what changes (curvature/thickness) and the consequence (focal length decreases) to secure full marks.
- Don't just say "lens changes shape" — be specific: curvature increases, lens becomes thicker, focal length decreases.