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Colour blindness

In most cases, colour blindness is an inherited condition. The retina at the back of the eye contains receptor cells called cones that are sensitive to red, green or blue light. People with colour blindness have a lack of receptors, or defects in them. People with red-green colour blindness, for example, have difficulty distinguishing shades of red and green.

There is currently no cure for inherited colour blindness. Some people use coloured filters to make some colours stand out, but other people report these to be more confusing.

Short and long sightedness

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 (bend) the light rays so that they do focus on the retina.

Short sight

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

Short sight 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 sight

Someone who is long-sighted 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 such as hard and soft contact lenses, laser surgery to change the shape of the cornea and a replacement lens in the eye. Contact lenses – work by being in 'contact' with the 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 lens – implanting artificial lenses is a recent development, and they can be placed in front of the original lens, through a small cut in the cornea, to correct an eye defect.