The table below shows results from an investigation of the pupil reflex. A volunteer sat in a dark room, then a lamp was switched on at different distances ...
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The table below shows results from an investigation of the pupil reflex. A volunteer sat in a dark room, then a lamp was switched on at different distances from one eye. A camera measured the diameter of the pupil. Fig. 1 shows the eye in cross-section. The iris contains muscles that alter the size of the pupil. Light-sensitive receptor cells in the retina send impulses to the brain, and motor neurones then carry impulses to muscles in the iris. The same person was used throughout so that age and eye colour did not affect the comparison.
| Distance of lamp from eye / cm | Pupil diameter / mm |
|---|---|
| 100 | 5.8 |
| 50 | 4.2 |
| 25 | 2.9 |
| 10 | 1.7 |
(a) What are the structures labelled X, Y and Z? [3]
(b) Describe the change in pupil diameter as the lamp is moved nearer to the eye. [2]
(c) Give the pupil diameter when the lamp is 25 cm from the eye. [1]
(d) Explain how this response protects receptor cells in the retina. [2]
Answer Details
Pupil reflex
- X is the iris, Y is the pupil and Z is the retina. [3]
- As the lamp moves nearer, the light becomes brighter and pupil diameter decreases. The values fall from 5.8 mm at 100 cm to 1.7 mm at 10 cm. [2]
- At 25 cm, the pupil diameter is 2.9 mm. [1]
- The iris makes the pupil smaller, so less light enters the eye. This protects receptor cells in the retina from damage caused by very bright light. [2]
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