A caver with normal vision must choose between two route markers that have the same shape but different colours.
The table shows the minimum light intensity needed for two groups of retinal receptor cells to respond.
| receptor cell group | minimum light intensity needed for a response in arbitrary units |
|---|---|
| P | 0.5 |
| Q | 20.0 |
Before a lamp is switched on, the light intensity at the caver's eye is 4.0 arbitrary units. Switching on the lamp increases the light intensity to 50.0 arbitrary units.
Use the data to explain why the caver cannot tell the two markers apart before the lamp is switched on and how switching on the lamp changes what the caver can see.
102 exam-style questions on Edexcel GCSE Biology 2.4 The eye, covering 2.4.1 Structure and function of the eye and 2.4.2 Eye defects and their correction. Each one has a worked solution and a mark scheme showing where the marks go.