Most myopia is caused by a natural change in the shape of the eyeball and occurs when light rays entering the eye focus in front of the retina, instead of directly on it. The eyeball of a nearsighted person is slightly longer than the eyeball of a normal vision person. Nearsightedness usually runs in families and often appears in childhood. Some times nearsightedness happens because of another disease or condition. See a doctor to learn the cause and treat the condition.
Attribution: Straight Talk Foundation