Describe how the pupil of the eye responds to a sudden increase in light intensity, naming the muscles involved and explaining the function of this response.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. The circular (sphincter) muscles of the iris contract.
2. The radial (dilator) muscles relax.
3. The pupil constricts, becoming smaller, so less light enters the eye.
4. This protects the retina from damage by bright light.
2. The radial (dilator) muscles relax.
3. The pupil constricts, becoming smaller, so less light enters the eye.
4. This protects the retina from damage by bright light.
Examiner tips
- Use the exact terms ‘circular (sphincter) muscles’ and ‘radial (dilator) muscles’.
- Show the sequence: contraction → relaxation → constriction → protection.
- Include the reason for the response – protection of the retina.
Common mistakes
- Confusing the muscles (e.g., saying the dilator contracts).
- Omitting the protective function of the response.
Mark scheme (4 marks)
- Circular muscles contract
- Radial muscles relax
- The pupil constricts / becomes smaller, so less light enters the eye
- This prevents damage to the eye / retina from bright light
Key terms in this question
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →