The eye is a sense organ that detects light from the surroundings.
|Sclera||White, outside layer||Protects the eye|
|Cornea||Transparent part of the sclera||Focuses light onto the lens|
|Iris||Coloured structure with muscles||Controls pupil size|
|Pupil||Hole in the iris||Allows light into the eye|
|Lens||Flexible transparent disc||Focuses light onto the retina|
|Retina||Lining of back of eye||Contains light-sensitive receptors|
|Fovea||Small part of retina with high receptor density||Sharp, clear vision|
|Choroid||Black-coloured layer||Prevents reflection from back of the eye|
|Optic nerve||Bundle of nerve fibres||Delivers information to the brain|
The retina is the light-sensitive layer of the eye. It contains two types of light-sensitive receptor cells:
- Rod cells are mainly used in low light levels.
- Cone cells allow for colour vision.
Rod cells cannot distinguish colour. However, they are roughly 100 times more sensitive to light than cones! This makes them most useful at low light levels (e.g. at sunset).
The human eye contains roughly 120 million rods and only 6 million cones.
There are three types of cone cell that absorb either red, green or blue light. Cones are most active at high light levels (e.g. on a sunny day).
The brain computes signals coming from red, blue and green cones to produce all the other colours you can see!
The fovea contains a higher density of cones than the rest of the eye. This allows objects you are looking at to be seen in high resolution.
The eye adjusts the thickness of the lens to focus on objects at different distances.
Focusing refers to directing light entering the eye onto the fovea.
Your eyes cannot focus on all objects in the environment at once. The eye has to adapt to focus on an object at a certain distance away. This is called accommodation.
Accommodation is controlled by ciliary muscles that are attached to the lens. When an object is far away, ciliary muscles relax, flattening the lens.
When an object is nearby, ciliary muscles contract, thickening the lens. This increases the amount the light is bent, focusing light onto the fovea.
The pupil adapts its size in response to different light intensities.
Your eyes need to control the amount of light entering the eye. Too little light makes vision grainy. Too much light can be damaging to light-sensitive cells.
|Low light||Bright light|
| || |
|Radial muscles contract||Radial muscles relax|
|Circular muscles relax||Circular muscles contract|
|Pupil dilates||Pupil constricts|
|More light enters eye||Less light enters eye|
Pupil size is affected by more than just light intensity. Hormones such as adrenaline can also cause pupils to dilate!