Question

a person cannot see clearly distance more than 40 cm. how much power of lens he should be advised to use

Answer 1

Thanks for asking
Hope it will help

Answer 2

The power of the lens is -2.5 D.

Explanation:

Given that,

A person cannot see clearly distance more than 40 cm.

Image distance = -40 cm

Object distance = ∞

We need to calculate the focal length

Using formula of the focal length

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

Where, u = object distance

f = focal length

v = image distance

Put the value in to the formula

$\dfrac{1}{f}=\dfrac{1}{-40}-\dfrac{1}{\infty}$

$f=-40\ cm$

We need to calculate the power

Using formula of power

$P=\dfrac{100}{f}$

Put the value into the formula

$P=\dfrac{100}{-40}$

$P=-2.5\ D$

Hence, The power of the lens is -2.5 D.

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Topic : Power of lens

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