What is myopia?What are its causes ?Name the type of the lens to correct this defect.

Explanation

Myopia (Short-sightedness) : A person suffering from myopia can see the nearby objects clearly but cannot see the distant objects so distinctly.
In a myopic eye, the image of distant object is formed in front of retina and not on the retina itself, as a result the images of distant objects appear blurred. The eye.s far point gets shifted from infinity to a point P.
Causes of Myopia : This defect arises due to either of the following two reasons :
(i) The elongation of the eyeball i.e., the distance of retina from the eye lens gets increased.
(ii) Excessive curvature of the cornea i.e., the focal length of the eye lens becomes small and does not increase sufficiently to accommodate infinity.
Correction of myopia : This defect can be corrected by using a concave (diverging) lens of appropriate focal length (or power). A concave lens forms the image of the distant object at the far point Pof the defective eye and thus forms the image of the object on the retina itself as shown in Fig.
Obviously, the focal length of the divergent lens required to correct the vision is equal to the distance of the far point P of the myopic eye.
(i) Normal eye : This image is formed on the retina.

(ii) Myopic eye: (a) The image is formed in front of retina, is the object lies far away. So the object is not seen clearly.

(b) Myopic eye : The image is formed at the retina if the object lies at far point of myopic eye, So the object is seen clearly.