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Concept Explanation
 

Defects Of Vision And Their Correction

Defects of Vision And Their Correction:

Myopia or Short-sightedness: In such a defect, one can see the nearby objects cleariy but it is difficult to see the objects at long distances. In such a defective of cye the image of distant objects is formed front of the retina at not at retina. This can be corrected by using a concave (divergent) lens.

Hypermetropia or long-sightedness: In such a defect, the distant objects are seen clearly, whereas the nearby objects look blurred. In this case, the image is formed behind the retina. This defect is known as long-sightedness or hypermetropia. It can be corrected by using a convex lens (converzing lens) of appropriate focal length.

Astigmatism: This defect is due to the cornea not being spherical. As a result, the rays of light entering the eye in different planes are brought to a focus at varying distances from the retina. So, through the object is a point, the shape of the image may be a line, a circle or any other shape except a point. This defect can be corrected by using a cylindrical lens, instead of a spherical lens.

Presbyopia: It is usually due to advancing age of the humans. In this condition, the lens becomes less flexible and near point recedes from the eye. As a result, the objects far away cannot be seen. This defect can be corrected by using a converging lens.

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