Explain how the eye focuses on a distant object and then refocuses to view a nearby object clearly. Include the role of the ciliary muscles and the lens in your answer.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
When viewing a distant object the ciliary muscles are relaxed, so the suspensory ligaments are taut and pull the lens into a flat shape. A flat lens has a long focal length and low optical power, allowing parallel rays from a distant object to be focused on the retina.
To view a nearby object the ciliary muscles contract, slackening the suspensory ligaments. This lets the lens become thicker and more curved, increasing its optical power and shortening its focal length so that the diverging rays from the near object are brought to a focus on the retina.
To view a nearby object the ciliary muscles contract, slackening the suspensory ligaments. This lets the lens become thicker and more curved, increasing its optical power and shortening its focal length so that the diverging rays from the near object are brought to a focus on the retina.
Examiner tips
- Use the word "relaxed" for distant view and "contract" for near view
- Mention the suspensory ligaments and their tension change
- Explain the change in lens curvature and focal length
Common mistakes
- Confusing the direction of muscle action (contract vs relax)
- Forgetting to mention the suspensory ligaments
- Not linking lens curvature to focal length change
Mark scheme (5 marks)
- When viewing a distant object, the ciliary muscles are relaxed
- The suspensory ligaments are taut / under tension, pulling the lens flat / thin
- A thin lens has a longer focal length / less power, so parallel rays from a distant object are brought to a focus on the retina
- To view a nearby object, the ciliary muscles contract
- The suspensory ligaments slacken / go slack, allowing the lens to become thicker / more curved, increasing its power / shorter focal length, so the image is focused on the retina
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →