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# A man is advised to wear lens as he can see objects between 15 cm and 30 cm. If he uses lens to see far off objects what will be his near point.

$(a)\;15\;cm \\ (b)\;30\;cm \\ (c)\;\frac{100}{15}\;cm \\ (d)\;\frac{10}{3} cm$

Focal length of lens required for the man is
$u= \infty \qquad v= -30\;cm$
$\large\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\large\frac{1}{f} =\frac{1}{-30}-\frac{1}{\infty}$
$f= -30 \;cm$
If he is wearing the lens,his near point is
$\large\frac{1}{-30}=\frac{1}{-15} -\frac{1}{u}$
$u= 30 cm$
Hence b is the correct answer.

edited Jul 15, 2014