If you have perfect vision, the light rays entering the pupils of your eyes will converge exactly on the retina at the back, and the sharply focused picture will be relayed to the brain.
Most people’s vision is at its sharpest at about the age of one year. Problems often develop at around puberty. The eyeball grows too long from front to back, or not long enough or it becomes misshapen. These are the three most common reasons why pepole need to wear glasses to correct the eye’s focal length.
In early life, a person’s potential sight problems may be compensated for by the strong action of the ciliary muscles attached to the iris of the eye’s lens. These muscles increase or decrease the curvature of the lens so that it is possible to focus on things up or far away.
But if the ciliary muscles weaken as often happens in middle age, the lens can no longer be made thick enough to focus on close-up objects, such as small type on a printed page. A person who at the age of ten could focus on the tip of his own nose may suddenly find that he cannot read a book unless he holds it at arm’s length.
The three main causes of blurred vision are long-sightedness, short-sightedness and astigmatism, and glasses of different types are used to correct them. Tinted lenses help wearers whose eyes are sensitive to light or reflections from clear lenses. An anti-reflection substance can also be used to coat lenses and help to make vision sharper.
Spectacle lenses are made of either glass or plastic. Glass is heavier but it also more resistant to scratching.
Picture Credit : Google