If our eye is not exactly of the correct shape, or the lens cannot focus properly, a clear image is not formed on the retina. Then, we need to wear glasses to correct our vision.
For a short sighted person, a distant object appears blurred because the image is formed in front of the retina.
But, in case of a long sighted person, the image is formed behind the retina, when he is trying to see a nearby object making it appear blur.
As people get older, the lenses of their eyes grow harder and cannot change their shape to change focus. Hence, glasses are required to have a correct vision.
Below Glasses info from Wikipedia (Read More):-
Glasses (also called eyeglasses or spectacles) are frames bearing lenses worn in front of the eyes, normally for vision correction, eye protection, or for protection from UV rays.
Modern glasses are typically supported by pads on the bridge of the nose and by temples placed over the ears. Historical types include the pince-nez, monocle, lorgnette, and scissors-glasses.
Eyeglass lenses are commonly made from plastic, including CR-39 and polycarbonate. These materials reduce the danger of breakage and weigh less than glass lenses. Some plastics also have more advantageous optical properties than glass, such as better transmission of visible light and greater absorption of ultraviolet light.[1] Some plastics have a greater index of refraction than most types of glass; this is useful in the making of corrective lenses shaped to correct various vision abnormalities such as myopia, allowing thinner lenses for a given prescription. Newer plastic lenses, called izon, can also correct for the higher order aberrations that naturally occur in the surface of our eye.[citation needed] These lenses create sharper vision for the people who have problems with sight and help with the halos, starbursts, and comet-tails often associated with night time driving glare.[citation needed] Wavefront guided LASIK surgery also corrects for the higher order aberrations.
http://en.wikipedia.org/wiki/Glasses
