During normal vision, light passes through the cornea, the clear covering of the eye, and then through the pupil, which is actually a hole in the colored part of the eye, or the iris. Light then passes through the lens, which focuses the image on the retina at the back of the eye. At this point the image is converted into electrical signals that are sent to the brain.
A person who is nearsighted (myopia) sees nearby objects clearly, but because the length of the eye is too long, the images of distant objects are focused in front of the retina, making the images blurred.
A farsighted person (hyperopia), sees far away objects clearly and nearby objects appear blurry because the lens focuses the images at a point behind the retina.
A physician can diagnosis these conditions with a standard ophthalmic examination.