A man cannot see clearly the objects beyond a distance of 20 cm from his eyes. To see distant objects clearly the kind of lenses and its focal length must be

100 cm convex

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100 cm concave

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20 cm convex

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20 cm concave

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Solution

The correct option is D 20 cm concave
$u = + 20 c m$
$v = \infty$
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
$\frac{1}{\infty} - \frac{1}{+ 20} = \frac{1}{f}$
$f = - 20 c m$

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A man cannot see clearly the objects beyond a distance of 20 cm from his eyes. To see distant objects clearly the kind of lenses and its focal length must be

The correct option is D 20 cm concaveu=+20cmv=∞1v−1u=1f1∞−1+20=1ff=−20 cm

The correct option is D 20 cm concave
$u = + 20 c m$
$v = \infty$
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
$\frac{1}{\infty} - \frac{1}{+ 20} = \frac{1}{f}$
$f = - 20 c m$