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Question: A person who can see things most clearly at a distance of 10 cm. requires spectacles to enable to hi...

A person who can see things most clearly at a distance of 10 cm. requires spectacles to enable to him to see clearly things at a distance of 30 cm. What should be the focal length of the spectacles

A

15 cm (Concave)

B

15 cm (Convex)

C

10 cm

D

0

Answer

15 cm (Concave)

Explanation

Solution

For lens u = wants to see = – 30 cm

and v = can see = – 10 cm

=1−10−1(−30)= \frac { 1 } { - 10 } - \frac { 1 } { ( - 30 ) } ⇒f=−15 cm\Rightarrow f = - 15 \mathrm {~cm}