A person enters a dark cinema after being outside in bright sunlight. At first they cannot see clearly, but after a few minutes their vision improves. Explain how the eye responds to the change in light intensity when the person first enters the dark cinema, and why their vision improves over time.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The pupil is the structure that controls how much light enters the eye.
In low or dim light the circular muscles of the iris relax and the radial muscles contract, causing the pupil to dilate and become larger, which allows more light to enter the eye.
The retina contains receptor cells – rods and cones – that detect light.
Rods are more sensitive to low light levels than cones.
Over time the rod cells become more sensitive to the low light, so more nerve impulses are sent to the brain via the optic nerve, improving vision.
In low or dim light the circular muscles of the iris relax and the radial muscles contract, causing the pupil to dilate and become larger, which allows more light to enter the eye.
The retina contains receptor cells – rods and cones – that detect light.
Rods are more sensitive to low light levels than cones.
Over time the rod cells become more sensitive to the low light, so more nerve impulses are sent to the brain via the optic nerve, improving vision.
Examiner tips
- Use the exact terms pupil, iris, rods, cones, optic nerve; show the sequence of pupil dilation then rod sensitivity.
- Explain both the mechanical change (pupil dilation) and the physiological adaptation (rod sensitivity).
Common mistakes
- Confusing the roles of the iris and pupil; describing the pupil as the light‑controlling structure only.
- Forgetting to mention rods as the primary cells for low‑light vision.
- Not explaining that rod sensitivity increases over time, leading to improved vision.
Mark scheme (5 marks)
- The pupil is the structure that controls how much light enters the eye
- In low/dim light the circular muscles of the iris relax and the radial muscles contract
- This causes the pupil to dilate/widen/become larger, allowing more light to enter
- The retina contains receptor cells (rods and cones) that detect light
- Over time the rod cells become more sensitive to the low light levels, so more nerve impulses are sent to the brain via the optic nerve, improving vision
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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