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Question

A person cannot see distinctly objects kept beyond2m. This defect can be corrected by using a lens of power


A

+ 0.5 D

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B

– 0.5 D

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C

+ 0.2 D

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D

– 0.2 D

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Solution

The correct option is B

– 0.5 D


Step (1): Given:

The far point of the person, D=2m

This means the person cannot see the objects situated beyond 2 m from the eye.

Step (2): Statement of Myopia:

Short-sightedness is an eye defect that causes the person to see far away objects as blurry or out of focus. The scientific term in use for short-sightedness is myopia.

From the given situation, the person is suffering from short-sightedness (or) myopia. A concave lens is used to correct myopia.

The far point of the given myopic eye, D is at 2 m.

Step (3): Finding the focal length of the lens:

Let the object distance u=-and image distance (v) = distance of far point = -D
Let 'f' be the focal length of the bi-concave lens.
Using the lens formula,1f=1v-1u

1f=1-121f=-12f=-2m

Step (4): Finding the power of the lens:

We know that the required power of the lens,

P=1fBysubstitutingthegivenvalue,wegetP=1-2=-0.5D

A concave lens of power of -0.5 D is required to fix the myopic eye.

Hence, the correct answer is (B) – 0.5 D


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