Structure and function of the eyeSpec 2.15B
The eye is a sensory receptor organ that detects light. Light is refracted (bent) so that it is focused on the retina, which contains the receptor cells.
| Part | Function |
|---|---|
| Cornea | Refracts (bends) light as it enters the eye |
| Lens | Refracts light to focus it on the retina |
| Iris | Changes the size of the pupil, so controls how much light enters the eye |
| Rod cells | Sensitive to dim light. They do not detect colour |
| Cone cells | Detect colour and work in bright light |
| Retina | Contains the rod and cone cells. Impulses go to the brain along the optic nerve |
The iris
The iris is the coloured ring of muscle around the pupil. Its muscles change the size of the pupil. In bright light the pupil gets smaller, so less light enters and the retina is protected from damage. In dim light the pupil gets larger, so more light enters and you can see better.
Rods and cones
Rod cells work in dim light but only give black and white vision. There are three types of cone cell, sensitive to red, green or blue light. Together they give colour vision. Cones need brighter light.
Tap to enlargeDefects of the eyeSpec 2.16B
| Defect | What happens |
|---|---|
| Cataracts | The lens becomes cloudy, so less light reaches the retina. Vision is blurred and can be lost. |
| Long-sightedness | The person can see distant objects clearly but not near objects. Light from near objects is focused behind the retina. |
| Short-sightedness | The person can see near objects clearly but not distant objects. Light from distant objects is focused in front of the retina. |
| Colour blindness | The person has difficulty telling some colours apart, such as red and green, because one or more types of cone cell are missing or not working properly. |
Long-sightedness happens when the eyeball is too short, or the lens and cornea refract light too little. Short-sightedness happens when the eyeball is too long, or the lens and cornea refract light too much.
Correcting eye defectsSpec 2.17B
| Defect | Correction |
|---|---|
| Long-sightedness | A convex lens (glasses or contact lenses) bends light inwards before it enters the eye, so it focuses on the retina instead of behind it. |
| Short-sightedness | A concave lens (glasses or contact lenses) spreads the light out before it enters the eye, so it focuses on the retina instead of in front of it. |
| Long- or short-sightedness (other option) | Laser eye surgery changes the shape of the cornea, so that light is focused on the retina. |
| Cataracts | Surgery removes the cloudy lens and replaces it with an artificial lens. |
Tap to enlargeQuick check
What does the iris do?
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It controls how much light enters the eye.
What is the difference between rod and cone cells?
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Rods work in dim light and do not detect colour. Cones detect colour and work in bright light.
What causes cataracts?
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The lens becomes cloudy, so less light reaches the retina.
What type of lens corrects short-sightedness?
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A concave lens.
Why can some people not tell red and green apart?
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One or more types of cone cell are missing or not working properly.