A person can see clearly only up to a distance of $25 c m$ He wants to read a book placed at a distance of $50 c m$ . What kind of lens does he require for this purpose and what must be its power?

Concave,

- 1.0 D

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Convex,

+ 1.5 D

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Concave,

- 2.0 D

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Convex,

+ 2.0 D

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Solution

The correct option is B Concave, $- 2.0 D$
For correcting myopia, concave lens is used and for lens.
$u =$ wants to see $= - 50 c m$
$v =$ can see $= - 25 c m$
From $\frac{1}{f} = \frac{1}{v} - \frac{1}{u} \Rightarrow \frac{1}{f} = \frac{1}{- 25} - \frac{1}{(- 50)} \Rightarrow f = - 50 c m$
So power $P = \frac{100}{f} = \frac{100}{- 50} = - 2 D$

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A person can see clearly only up to a distance of 25 cm He wants to read a book placed at a distance of 50 cm. What kind of lens does he require for this purpose and what must be its power?

The correct option is B Concave, −2.0 DFor correcting myopia, concave lens is used and for lens.u= wants to see =−50 cmv= can see =−25 cmFrom 1f=1v−1u⇒1f=1−25−1(−50)⇒f=−50 cmSo power P=100f=100−50=−2 D

A person can see clearly only up to a distance of $25 c m$ He wants to read a book placed at a distance of $50 c m$ . What kind of lens does he require for this purpose and what must be its power?