The eye
Our eyes are specialised sense organs that contain Organ, tissue or cell that detects a stimulus. sensitive to light.
There are many parts that work together to produce a focused image on the The light-sensitive area on the back of the eye. Light must be focused on the retina in order to see clearly. under different light conditions.
| Structure | Function |
| Conjunctiva | Thin, transparent protective outer covering that prevents entry of microorganisms. |
| Cornea | Transparent part at front of eye that allows light to enter. It also causes slight bending/refraction of light. |
| Pupil | Space in the iris that allows light into the eye. |
| Iris | Controls how much light enters the eye by changing its diameter. |
| Lens | Bends/refracts light towards the retina. |
| Aqueous humour | Fluid in front of the lens that helps maintain the shape of the eye and lens. |
| Vitreous humour | Fluid behind the lens that helps maintain the shape of the eye and lens. It also keeps the retina against the wall of the eye. |
| Retina | Contains light sensitive receptor cells. |
| Optic nerve | Contains neurones that carry information from the eye to the brain. |
| Structure | Conjunctiva |
|---|---|
| Function | Thin, transparent protective outer covering that prevents entry of microorganisms. |
| Structure | Cornea |
|---|---|
| Function | Transparent part at front of eye that allows light to enter. It also causes slight bending/refraction of light. |
| Structure | Pupil |
|---|---|
| Function | Space in the iris that allows light into the eye. |
| Structure | Iris |
|---|---|
| Function | Controls how much light enters the eye by changing its diameter. |
| Structure | Lens |
|---|---|
| Function | Bends/refracts light towards the retina. |
| Structure | Aqueous humour |
|---|---|
| Function | Fluid in front of the lens that helps maintain the shape of the eye and lens. |
| Structure | Vitreous humour |
|---|---|
| Function | Fluid behind the lens that helps maintain the shape of the eye and lens. It also keeps the retina against the wall of the eye. |
| Structure | Retina |
|---|---|
| Function | Contains light sensitive receptor cells. |
| Structure | Optic nerve |
|---|---|
| Function | Contains neurones that carry information from the eye to the brain. |
Focusing the eye
As light passes through the cornea some The bending of light as it passes from one substance into another (also happens with sound waves). takes place. More refraction occurs when the light passes through the lens.
The thickness of the lens can be adjusted in order to focus light on the retina.
When looking at a distant object the lens is thin as little adjustment is needed to focus light onto the retina.
When looking at nearby objects the lens is thick to provide additional refraction to focus light on the retina.
Image caption, Looking at a distant object
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Accommodation - the lens changing shape
A circular ring of muscle, known as the ciliary muscle, surrounds the lens.
The lens is attached to the ciliary muscle by suspensory ligaments.
When the ciliary muscle relaxes, its The length of a line that runs from one edge of a circle to another, passing through the centre. becomes wider; the suspensory ligaments tighten and pull the lens thinner.
This happens when focussing on a distant object.
When the ciliary muscle contracts, its diameter becomes smaller; the suspensory ligaments slacken causing the lens to go back to its normal thicker shape.
This happens when focussing on a nearby object.
Controlling the amount of light entering the eye
Too little or too much light can damage the light sensitive receptor cells in the retina.
In dim light, the pupil becomes larger to allow as much light as possible to enter.
In bright light, the pupil becomes smaller to restrict the amount of light entering.