A far sighted person has his near point 50 cm. Find the power of lens he should use to see at 25 cm clearly.

+1 D

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+2 D

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-2 D

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-1 D

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Solution

The correct option is D +2 D
In order for a person to see the object at 25cm clearly, the lens should make an image of the object at 25cm distance at a distance of 50 cm.
Hence, $u = - 25 c m = - 0.25 m$
$v = - 50 c m = - 0.5 m$
Thus, Power $= \frac{1}{f} = \frac{1}{v} - \frac{1}{u} = - 2 D + 4 D = + 2 D$

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A far sighted person has his near point 50 cm. Find the power of lens he should use to see at 25 cm clearly.

The correct option is D +2 DIn order for a person to see the object at 25cm clearly, the lens should make an image of the object at 25cm distance at a distance of 50 cm.Hence, u=−25cm=−0.25mv=−50cm=−0.5mThus, Power =1f=1v−1u=−2D+4D=+2D