Question

A person cannot see beyond 2 m. If he has to read a signboard 4 m away, then the power of the lens required is

• A. 0.25 D
• B. -0.25 D
• C. 0.75 D
• D. -0.75 D

Answer 1

The correct option is B -0.25 D

As the person cannot see beyond 2 m, the image of the object at 4 m must form at 2 m from the lens.
∴ Image distance, v = -2m
Object distance, u = -4m
Using lens formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\Rightarrow \frac{1}{-2}-\frac{1}{-4}=\frac{1}{f}$
$\Rightarrow \frac{1}{f}=\frac{1}{-4}=-0.25m^{-1}$
$\Rightarrow \text{Power,}P=\frac{1}{f}=-0.25D$
Hence, the correct answer is option (b)