Question

A man can't see beyond 80cm. What's the power of lens he should use ?​

Answer 1

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As the person cannot see objects closer than 80 cm, hence for all objects the lens should make an image beyond 80 cm. Now the image is in front of the lens hence it is taken as negative. ... This defect is called hypermetropia rectified by using convex lens

Answer 2

The power of the lens is -0.625 D and he should use a concave lens.

Explanation:

Suppose, A man can't see beyond 80 cm from his eye while a person with normal eyesight can see object easily placed up to 160 cm .

We need to calculate the focal length of the lens

Using formula of lens

$\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}$

Where, v = image distance

u = object distance

f = focal length

Put the value into the formula

$\dfrac{1}{f}=\dfrac{1}{-80}-\dfrac{1}{-160}$

$\dfrac{1}{f}=-\dfrac{1}{160}$

$f=-160\ cm$

We need to calculate the power of the lens

Using formula of power

$P=\dfrac{100}{-160}$

$P=-0.625\ D$

Hence, The power of the lens is -0.625 D and he should use a concave lens.

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Topic : optics

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