Question

A person cannot see nearer than 1.5 m. If he has to read a signboard 40 cm away, then the power of the lens required is

• A. 0.25 D
• B. 1.83 D
• C. 0.75 D
• D. –0.75 D

Answer 1

The correct option is B 1.83 D

As the person cannot see nearer than 1.5 m, the image of the object should be formed by the lens at 1.5 m so the object can be seen clearly by the eye.
∴ Object distance, u = –40 cm = –0.4 m
Image distance, v = –1.5 m
As per Lens formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$Powerofthelens,P=\frac{1}{f}=\frac{1}{(-1.5)}-\frac{1}{(-0.4)}$

= 1.83 D
Hence, the correct answer is option (b).