91�ȱ�

Correcting vision defects

Two common defects of the eyes are myopia (short-sightedness) and hyperopia (long-sightedness). In both cases rays of light do not focus on the retina so a clear image is not formed.

These two defects are treated with spectacle lenses, which refract the light rays so that they do focus on the retina.

Short-sightedness

Someone with short-sightedness can see near objects clearly, but cannot focus properly on distant objects.

Short-sightedness is caused by one of the following:

• the eyeball being elongated - so that the distance between the lens and the retina is too great
• the lens being too thick and curved - so that light is focused in front of the retina

Short-sightedness can be corrected by placing a concave lens in front of the eye, as shown in the diagrams below.

Long-sightedness

Someone with long-sightedness can see distant objects clearly, but they cannot focus properly on near objects.

Long-sightedness is caused by one of the following:

• the eyeball being too short - so the distance between the lens and retina is too small
• a loss of elasticity in the lens - meaning it cannot become thick enough to focus (which is often age-related)

As a result, the lens focuses light behind the retina instead of onto it. Long-sightedness is corrected by putting a convex lens in front of the eye, as shown in the diagrams below.

New technologies have provided alternatives to wearing spectacle lenses.

Contact lenses – work by being in 'contact' with your eye. They float on the surface of the cornea. They work like spectacle lenses, by focusing and refracting the light.

Laser surgery – reshapes the cornea surgically. Common for myopia but can be used for some hyperopia conditions.

Replacement lenses – implanting artificial lenses is a recent development. The lens is placed in front of the original eye lens.