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Question

A person with a myopic eye cannot see objects beyond 1.2m distinctly. What should be the type of corrective lens used to restore proper vision?


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Solution

Step 1. Formula to find the power of lens

In a normal eye, the distant point is infinity. The lens used should be such that an object at infinity, forms a virtual image at 1.2m.

Formula to find the power of lens is,

P=1f

Here, P is the power of the lens and f is the focal length.

We know that the relation,

1f=1v-1u

Here, v is the image distance and u is the object distance.

Step 2. Calculate the power of the corrective lens used to restore proper vision.

It is given that, a person can't see objects beyond 1.2m directly.

So, in order to view the distances beyond 1.2m, from the given,

v=-1.2m

u=-

f=?

Put the values in the above relation, we get,

1f=1-1.2-1-

1f=-11.2

f=-1.2m

Put the value of focal length in the power of lens formula, we get,

P=1-1.2

=-56

=-0.83D

So, the person suffers from Myopia, the corrective lens will be concave or diverging lens.

Hence, the power is -0.833D should be used to restore proper vision.


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