Question

A person sees clearly at a distance of 100 cm, then power of lens used to see object at 40 cm is

• A. 3 D
• B. - 3 D
• C. - 1.5 D
• D. + 1.5 D

Answer 1

Answer: (D)

+ 1.5 D

Answer 2

by lens formula $\frac{1}{f} = \frac{1}{v}-\frac{1}{u}$

Given $v = - 100\, cm$ and $u = - 40\, cm$
$\therefore \frac{1}{f} = -\frac{1}{100}-\frac{1}{-40}$
$\Rightarrow \frac{1}{f} = -\frac{1}{100}+\frac{1}{-40}$
$\Rightarrow \frac{1}{f} = \frac{-1+2.5}{100}+\frac{1.5}{100}$
Hence, power of lens is
$P = \frac{100}{f (cm)} = 100 \times \frac{1.5}{100}=1.5\, D$