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