Nearsightedness, also called shortsightedness or myopia, is caused by a defect in the shape of the eye. People with myopia have eye-balls which are longer from front to back than the normal eye. They can see nearby objects without difficulty, but distant objects appear to be blurred to such people. Special glass or plastic lenses can correct the seeing difficulty.
The lens in the eye is similar in some respects to a camera lens because both gather in light rays reflected from objects and bring them to a focus upon a sensitive surface— the retina of the eye or film in the camera. However, while the camera lens can be moved closer to the film for distant objects and farther from the film for objects nearby in order to focus clearly, the lens in the eye cannot move back and forth within the eye-ball, nor can the eyeball change in length in order to adjust.
The lens is held in position by the suspensory membrane and is attached to the ciliary muscle. When this muscle contracts, forcing the lens to become thicker and rounder, objects nearby are brought into focus. When it relaxes, the lens becomes flatter and distant objects are brought into focus. If the eyeball is too long, the lens can never become flat enough to compensate for the extra length, so the light rays entering the eye are brought to a focus before they reach the retina.
Eyeglasses with concave lenses can improve the vision of a person with myopia by bending the light rays in such a way that they can be brought into focus. Nearsightedness is believed to be an inherited defect.
