Why Vision Becomes Blurred

1. A sharp image is formed only when light is brought to a focus exactly on the retina.
2. If the light converges in front of the retina or is still converging behind it, the image on the retina is blurred.
3. This happens when the eyeball is the wrong length or when the cornea or lens refract by the wrong amount.
4. Most defects are corrected by placing a lens in front of the eye that changes the light before it reaches the cornea.

Short-Sightedness

1. A person with short-sightedness, also called myopia, can see near objects clearly but distant objects look blurred.
2. Light from a distant object is brought to a focus in front of the retina.
3. This happens because the eyeball is too long, so the retina sits further back than the focal point.
4. It can also happen because the cornea is too curved or the lens is too thick, so the light is refracted too much.
5. It often develops during childhood and adolescence as the eyeball grows, and it tends to run in families.

Correcting Short-Sightedness

1. Short sight is corrected with a concave lens, also called a diverging lens, which is thinner in the middle than at the edges.
2. The lens spreads the light rays out before they enter the eye.
3. The cornea and lens then bring them to a focus further back, so the focal point moves onto the retina.
4. The same correction can be worn as a contact lens, which sits directly on the cornea and gives a wider field of view.
5. Laser eye surgery corrects it permanently by removing tissue to flatten the cornea, so the cornea refracts the light less.
6. A replacement lens can be implanted inside the eye for a very severe case, though this carries a higher risk of infection and damage to the retina.

Long-Sightedness

1. A person with long-sightedness, also called hyperopia, can see distant objects clearly but near objects look blurred.
2. Light from a near object is brought to a focus behind the retina.
3. This happens because the eyeball is too short, so the retina sits in front of the focal point.
4. It can also happen because the lens is too thin or not curved enough, so the light is not refracted enough.
5. It becomes very common after about the age of 40, because the lens gradually loses its elasticity and can no longer become thick enough to focus on close work.

Correcting Long-Sightedness

1. Long sight is corrected with a convex lens, also called a converging lens, which is thicker in the middle than at the edges.
2. The lens brings the light rays together before they enter the eye.
3. The cornea and lens then bring them to a focus further forward, so the focal point moves onto the retina.
4. Contact lenses work in the same way, and reading glasses are simply convex lenses used only for close work.
5. Laser surgery can also be used, this time to make the cornea more curved so that it refracts light more strongly.

Cataracts

1. A cataract is a cloudy patch in the lens of the eye.
2. It forms as protein builds up in the lens, usually with age, and clumps together so the lens is no longer fully transparent.
3. Light is scattered rather than passing straight through, so the image on the retina is blurred and misty.
4. Colours look faded, bright lights produce a glare or a halo, and vision is worse at night.
5. The risk is increased by ageing, by long exposure to ultraviolet light, by smoking and by diabetes.
6. A cataract is corrected by surgery to remove the cloudy lens and replace it with an artificial plastic lens.

Colour Blindness

1. Colour blindness is a defect in which certain colours cannot be told apart.
2. It is caused by one or more types of cone cell in the retina not working properly or being absent.
3. Red green colour blindness is by far the most common form, and it affects the red or the green cones.
4. It is almost always inherited rather than developed, and it is much more common in males than in females.
5. There is no cure, because the faulty cone cells cannot be repaired or replaced.
6. Tinted lenses and filters can help some people tell certain colours apart, but they do not restore normal colour vision.

Comparing the Four Defects

1. Short-sightedness. Distant objects blur because light focuses in front of the retina, and a concave lens corrects it.
2. Long-sightedness. Near objects blur because light focuses behind the retina, and a convex lens corrects it.
3. Cataracts. All vision becomes cloudy because the lens scatters light, and replacing the lens corrects it.
4. Colour blindness. Colours cannot be distinguished because cone cells are faulty, and it cannot be corrected.