Question

The near point of a person is 75cm. In order that he may be able to read book at a distance 30cm. The power of spectacles lenses should be

• A. -2 D
• B. + 3.75 D
• C. +2 D
• D. + 3 D

Answer 1

The correct option is C +2 D

Hypermetropia $\Rightarrow$ Convex lens (converging lens)
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
v = -75cm, u = -30cm
$\frac{1}{f}=\frac{-1}{75}+\frac{1}{30}$
$\frac{1}{f}=\frac{-2+5}{150}=\frac{3}{150}=\frac{1}{50}$
f = 50cm
$P=\frac{100}{f\left(incm\right)}=\frac{100}{50}=2D$