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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 $

Can you answer this question?
 
 

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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.

 

answered Jan 21, 2014 by meena.p
edited Jul 15, 2014 by thagee.vedartham
 

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