What is the approximate area of the pupil when its diameter is:

- 8 mm?
- 2 mm?
- Approximately how many times as much light does the pupil let in when it is biggest as when it is smallest?

Solution:

- The radius of the pupil of 8 mm diameter is 4 mm. Therefore, the area of the pupil is: pi * radius * radius = pi * 4 * 4 = pi * 16 = 3.14 * 16 = 50.24. The answer is approximately 50 mm square.
- The radius of the pupil of 2 mm diameter is 1 mm. Therefore, the area of the pupil is: pi * radius * radius = pi * 1 * 1 = pi * 1 = 3.14 * 1 = 3.14. The answer is approximately 3 mm square.
- The ratio of the area of the pupil when it is biggest to the area when it is smallest = 50.24/3.14 = 16. So there is 16 times more light the pupil lets in when it is biggest as when it is smallest.