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 curved lenses focus images on the retina instead of behind it.
Some people wear glasses to correct other eye problems, and these glasses work differently. But most people who wear glasses wear them because they’re either.nearsighted or farsighted.
Two-thirds of all adults in America wear eyeglasses!