How Do Corrective Lenses Work?
When a person is myopic or nearsighted, the objects are focused in front of the retina instead of on it. This causes distant objects to appear blurry. Myopia is typically caused by the eyeball being too long. By placing a concave lens in front of the eye, the light rays are diverged so that the focal point is farther away.
Use of a concave lens to correct myopia. www.howstuffworks.com
Hyperopia, or farsightedness, is when the focal point is behind the retina. Hyperopic people can see distant objects fine, but can not focus on close objects. Convex lenses are used to shorten the distance of the focal point to bring it closer to the retina. This condition is usually caused by the eye ball being too short.
Use of a convex lens to correct hyperopia. www.howstuffworks.com