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: the 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 your eye. They float on the surface of the cornea. They work like spectacle lenses, by focusing and refracting the light.