Accommodation, rods and cones
Accommodation (Supplement)
- Accommodation is changing the lens shape to focus near or far.
- It uses the ciliary muscles and suspensory ligaments:
- near object: ciliary muscles contract, ligaments slacken, lens gets fatter (more refraction),
- far object: ciliary muscles relax, ligaments pull tight, lens gets thinner (less refraction).
Practice
To focus on a NEAR object, the lens becomes:
For a near object the ciliary muscles contract, the ligaments slacken and the lens becomes fatter (more refraction).
Rods and cones (Supplement)
| Receptor | Best for | Colour? |
|---|---|---|
| rods | dim light (night vision) — very sensitive | no |
| cones | bright light | yes — three kinds for colour |
- Cones are packed densely at the fovea, which gives the sharpest image.
Practice
Rods are receptor cells that:
Rods are very sensitive (good for night vision) but give no colour; cones give colour in bright light.
Practice
Cones give colour vision and are packed most densely at the fovea.
There are three kinds of cone for colour; they cluster at the fovea, which gives the sharpest image.
You've got it
Key idea
- accommodation (Supplement): near → fatter lens (ciliary muscles contract); far → thinner lens
- rods = dim light, no colour; cones = bright light, colour vision
- cones cluster at the fovea for the sharpest detail