Question

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

• A. + 1 D
• B. + 2 D
• C. - 2 D
• D. - 1 D

Answer 1

Answer: (B)

+ 2 D

Answer 2

Here $u = - 25 \,cm, v = -50 \,cm$ We have $\frac{1}{f}=\frac{1}{-u}+\frac{1}{v}$ i.e, $\frac{1}{f}=\frac{1}{25}-\frac{1}{50}$ or $f = 50\, cm$ Power of lens he should use, $P=\frac{100}{f}=\frac{100}{50}=+2D$