Glasses are made up of lenses and a frame. The frame can be made of plastic, metal or a combination of the two; its function is to keep the lenses positioned in front of your eyes. The part that sits on the bridge of your nose is called the "bridge", and the handles which fit on your ears are called "temples".
There are three types of lenses which are commonly used to correct vision problems - convex, concave, and cylindrical.
Concave lenses correct near sight, when the eye focuses light in front of the retina, and convex lenses correct far sight, where the eye's focus is behind the retina. A cylindrical lens can often correct astigmatism, caused by the front surface of the eye curving unevenly. Bifocal lenses have two areas, for distance vision at the top and near vision at the bottom.
History: Glasses first appeared in about AD 1300, but were little used until the mid-15th century, when the advent of printing and concomitant increase in literacy stimulated demand. By 1600 there was an optician in most European towns. Spectacle lenses are made from shatterproof plastic or glass; sometimes they are photochromic or coated to reduce reflections or protect the eye from ultraviolet light. They curve away from the eye to ensure accurate correction over the whole lens.