Defects of vision

The two main defects of vision are long sightedness and short sightedness.

• short-sighted - cannot focus on objects in the distance

The lens does not focus the light rays from far away objects exactly onto the retina. The light rays are focused just in front of the retina.

If the short-sighted person wears spectacles with a concave or diverging lens the light rays spread or diverge just before they reach the eye, and then are focused exactly on the retina.

Long sightedness is exactly the opposite.

• short-sighted - cannot focus on objects close by

The lens focuses the light rays from near by objects behind the retina.

A long-sighted person must wear spectacles which have a convex lens. This brings in, or converges, the rays before they reach the eye so that they focus exactly on the retina.