Short sight occurs when the eyeball is too long or the lens is too thick, or both. As a result, light rays from distant objects are focused in front of the retina (because the light rays are highly converged). The image formed on the retina is therefore out of focus.
Short sight can be corrected by wearing glasses with concave lenses. Light rays from distant objects are diverged by concave lenses before entering the eyes, so that light can be focused on the retina to form a sharp image.
