Figure below shows the shape of the eye that is suffering from short sightedness or myopia. In this case the image of an object placed at far off distance is formed in front of the retina. To correct this defect, a concave lens has to be worn by the person. The focal length of the concave lens is such that the parallel light rays coming from distant objects are first diverged a little and then are converged by the eye lens. The adjustment of the focal length of the concave lens has to be in such a manner that a well focused image is formed at the retina.
Some people suffer from both short as well as long sightedness.
In the natural eye, cornea and lens have inherent density defect because of which, a distant point object will be seen as a vertical, horizontal or inclined line. Complicated cylindrical lenses correct this defect. This is because to see clearly in the dark the inside of their eye is made of up materials which reflect light. Multiple reflection within the eye gives them a better vision in the dark. Some of the reflected light escapes through the pupil and hence the eyes of these animals glow in the dark.