Eye defects (AQA GCSE Biology): Revision Notes
Eye defects
What are eye defects?
Eye defects are problems with how the eye focuses light. The two most common eye defects are myopia (short-sightedness) and hyperopia (long-sightedness). These happen when light doesn't focus properly on the retina at the back of the eye.
The retina is the light-sensitive layer at the back of the eye that converts light into electrical signals sent to the brain. For clear vision, light must focus exactly on this layer.
Myopia (short-sightedness)
Myopia means you can see things that are close to you clearly, but things far away look blurred.
What causes myopia?
Myopia happens when light focuses in front of the retina instead of on it. This occurs because:
Common causes of myopia:
- The eyeball is too long
- The cornea (front part of the eye) is too curved
How is myopia treated?
Myopia is corrected using concave lenses in glasses or contact lenses. These lenses are thinner in the middle and thicker at the edges. They work by diverging (spreading out) light rays before they enter the eye. This means the light can focus properly on the retina.
How Concave Lenses Correct Myopia:
Step 1: Light rays from distant objects enter the concave lens Step 2: The lens diverges (spreads out) the light rays Step 3: The spread-out rays then enter the eye Step 4: The eye can now focus these rays correctly on the retina
Hyperopia (long-sightedness)
Hyperopia means you can see things that are far away clearly, but things close to you look blurred.
What causes hyperopia?
Hyperopia happens when light focuses behind the retina instead of on it. This occurs because:
Common causes of hyperopia:
- The eyeball is too short
- The cornea is too flat
How is hyperopia treated?
Hyperopia is corrected using convex lenses in glasses or contact lenses. These lenses are thicker in the middle and thinner at the edges. They work by converging (bringing together) light rays before they enter the eye. This helps the light focus properly on the retina.
How Convex Lenses Correct Hyperopia:
Step 1: Light rays from near objects enter the convex lens Step 2: The lens converges (brings together) the light rays Step 3: The converged rays then enter the eye Step 4: The eye can now focus these rays correctly on the retina
Other treatments for eye defects
Hard and soft contact lenses
Contact lenses sit directly on the eye and work the same way as glasses. They can be more convenient for sports and daily activities as they move with your eye and provide a wider field of vision.
Laser eye surgery
A laser changes the shape of the cornea so it bends light correctly. This can reduce or eliminate the need for glasses or contacts. The laser reshapes the cornea so light focuses properly on the retina.
Artificial lens implants
Sometimes an artificial lens is put into the eye. This can replace the natural lens or work alongside it. The artificial lens has a fixed shape, so glasses might still be needed afterwards for fine focusing.
Cataracts
A cataract happens when part of the eye's natural lens becomes cloudy or less transparent. This stops some light from reaching the retina, making vision blurry or cloudy.
Treatment for cataracts
Cataract Surgery Process:
- The cloudy natural lens is removed
- A clear artificial lens is put in its place
This is one of the most successful and common surgical procedures performed today.
Age and eye defects
As people get older, their eye muscles become weaker. This makes it harder for the eye to change shape and focus on near objects. This is why many older people need reading glasses.
This age-related condition is called presbyopia. It's a natural part of ageing that typically begins in the mid-40s, when the lens of the eye becomes less flexible.
Remember!
Key Points to Remember:
- Myopia = short-sightedness = can't see distant objects clearly
- Hyperopia = long-sightedness = can't see near objects clearly
- Concave lenses treat myopia by spreading light out
- Convex lenses treat hyperopia by bringing light together
- Cataracts make the lens cloudy and are treated with surgery