Question

A person can see clearly up to 3 metres what should be the power of the lens in his spectacles so that he could see clearly up to 12 metres.

• A. 0.25 D
• B. -0.25 D
• C. 0.5 D
• D. - 0.75 D

Answer 1

The correct option is B -0.25 D

Answer is B.
The lens equation is given as $\frac{1}{u}+\frac{1}{v}=\frac{1}{f}$
$\frac{1}{1200}+\frac{1}{-300}-\frac{1}{f}$ (u and v are in cm)

The negative sign is due negative value of the image distance of the concave lens.
$f=-400cm$
Power $=\frac{100}{f}$ in cm.
So, $\frac{100}{-400}=-0.25D$.
Hence, the power of the lens in his spectacles is -0.25D.