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Defects of Vision and Their Correction

A man can not...

Question

A man can not see clearly kept at less than $150 c m$ . He wants to read notice at distance $25 c m$ . What should be the nature and focal length of the lens used in spectacles?

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Solution

A) U = -25 cm
V = -150 cm
$\frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{30}$
f = 30 cm
$P = \frac{1}{f}$
$P = \frac{1}{30} = 0.03 c m = 0.0003 D$

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