Scientists take samples from two positions in the retina. They measure the percentages of the two types of receptor cell and the lowest light intensity at which each sample first produces electrical impulses.
Under the test conditions, each receptor type has the same response threshold at both positions.
| Position | Rod cells (%) | Cone cells (%) | Lowest light intensity that produces impulses (relative units) |
|---|---|---|---|
| P | 0 | 100 | 40 |
| Q | 88 | 12 | 4 |
A third sample is taken from the blind spot.
Explain why shining light on this sample does not produce electrical impulses.
Give the position most likely to be the fovea. Use the information in the table to give one reason for your answer.
A small blue symbol produces a light intensity of 6 relative units. Its image is first focused on position P and then on position Q.
Explain whether the symbol would be seen at each position and, if seen, what colour it would appear.
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.