distant objects cannot be seen clearly. The focal length of eye lens is reduced or the distance between eye lens and retina increases. Hence, the far point will not be infinity for such eyes and the far point has come closer. Due to this, the image of distant objects are formed before the retina (Figure .
-a). This defect can be corrected using a concave lens (Figure . -b). The focal length of the concave lens to be used is computed as follows: Figure .
(a) Vision with myopia b) Corrected vision using a concave lens (a) Myopia (b) Myopia corrected eye Let a person with myopia eye can see up to a distance x. Suppose that he wants to see all objects farther than this distance, i.e., up to infinity. Then the focal length of the required concave lens is f = –x. If the person can see up to a distance x and if he wishes to see up to a distance y, then, the focal length of the required concave lens is, f = xy x − y .
.( . ) Hypermeteropia Hypermeteropia, also known as long sightedness, occurs due to the shortening of eye ball. With this defect, distant objects can be seen clearly but nearby objects cannot be seen clearly. The focal length of eye lens is increased or the distance between eye lens and retina decreases.
Hence, the near point will not be at 25cm for such eyes and the near point has moved farther. Due to this, the image of nearby objects are formed behind the retina (Figure . -a). This defect can be corrected using a convex lens (Figure .
-b). The focal length of the convex lens to be used is computed as follows: Figure . (a) Vision with hypermeteropia (b) Corrected vision using a convex lens Let a person with hypermeteropia eye can see object beyond a distance d. Suppose that he wants to see all objects closer than this distance up to a distance D.
Then, the focal length of the required