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State causes of myopia. How is this defe...

State causes of myopia. How is this defect corrected ? Draw suitable diagram to show it.

Text Solution

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Causes of myopia :
(1) The focal length of the eye lens is decreased due to excessive curvature of the eye lens.
(2) The length of eyeball is elongated (longer than normal), i.e., the distance between the retina and the eye lens is increased.
Due to either or both the causes, the eye lens is not able to focus the rays coming from a distant object (at infinity) on the ratina.

Correction of the defect : To correct a myopic eye, the person has to wear spectacles with a concave (i.e., divergent) lens of suitable focal length or power.
So that parallel rays coming from infinity appear, after refraction through the lens, to come from the far point of myopic eye, when the eye is unaccommodated.
These parallel rays from a distant object are brought to the focus upon the retina with help of a concave lens.
Obviously, the focal length of the required lens is equal to the distance of the far point from the eye.
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