Accommodation
Focusing near and far
Light is refracted (bent) by the cornea and again by the lens so that it comes to a focus on the retina. To focus on objects at different distances, the lens changes shape. This is called accommodation.
25 cm1 m6 m
About reading distance
What you see
Sharp. Light focuses on the retina.
| When looking at | Near object | Distant object |
|---|
| Ciliary muscles | Contract | Relax |
|---|
| Suspensory ligaments | Slacken (go loose) | Tighten (pulled taut) |
|---|
| Lens | Fatter, more convex | Thinner, less convex |
|---|
| Light is refracted | More | Less |
|---|
Stops the lens changing shape. Freeze it, then move the object to see why accommodation matters.
Pupil reflex
Bright light, dim light
The iris controls how much light reaches the retina. It has two sets of muscles, circular and radial, that work as an antagonistic pair: when one set contracts, the other relaxes.
| In | Bright light | Dim light |
|---|
| Circular muscles | Contract | Relax |
|---|
| Radial muscles | Relax | Contract |
|---|
| Pupil | Constricts (gets smaller) | Dilates (gets wider) |
|---|
| Light entering | Less | More |
|---|
How the reflex happens
- 1Receptors in the retina detect the change in light
- 2A sensory neurone carries impulses to the brain
- 3A relay neurone in the brain passes the impulse on
- 4A motor neurone carries impulses to the iris
- 5The effectors, the iris muscles, respond and the pupil changes size
This is a reflex: fast and automatic, with no conscious thought. In bright light it stops the retina from being damaged. In dim light it lets in as much light as possible so you can still see.