The dark-adapted eye
- Pupil dilation to see faint objects may take up 20 minutes
- This can be spoilt in moments by looking at a light
- Red light has less effect, so use a dim red torch
- The light-sensitive cells in the human retina are of two kinds called rods and cones
- Rods are extremely sensitive to light but cannot distinguish colours
- The central part of the retina mainly contains cones - good for colour but not so sensitive
- So in the dark
- we cannot distinguish colour and
- we have to use averted vision to see faint objects
- Use a red torch to retain dark-adaptation, avoid all other lights