How do eyeglasses work?

Eyeglasses aid those whose eyesight is imperfect. Glasses are made of two lenses mounted in a frame so that one lens is in front of each eye. The lenses help to focus rays of light so that they will fall properly on the retina and form a clear image. If you can see distant objects but cannot focus well on close objects, your problem is called farsightedness. The eyeball is short and the focus of the light rays falls behind the retina. In the case of farsightedness, a convex glass is used in your eyeglasses. A convex lens, which has a surface that is curved or rounded outward, is used to slant the light rays together, or converge them. If you cannot focus rays which come from distant objects, this problem is called nearsightedness. The eye focuses the image so that it falls short of the retina. The image on the retina is blurred, like the image a motion picture projector puts on the screen when it is out of focus.  more

The retina is a kind of screen at the back of the eyeball. It is on the retina that the lens of the eye focuses the image of everything we see. But when a person is nearsighted, images are focused in front of the retina. The farther away an object is from the eye, the farther in front of the retina its image is focused. So a person who is nearsighted can see things that are close very well, but cannot see things clearly when they're at a distance. The lenses of glasses made for a nearsighted person are curved in a certain way so that they focus images on the retina instead of in front of it. When a person is farsighted, images are focused behind the retina. The closer an object is to the eye, the farther behind the retina its image is focused. A farsighted person can see things clearly when they're far away, but cannot see things that are very close, like pages of a book. So a farsighted person wears lenses that are curved the opposite way from lenses for nearsighted persons. These ...  more