Question

A man can see clearly up to 3 metres. Prescribe a lens for his spectacles so that he can see clearly up to 12 metres

• A. $-\frac{3}{4}D$
• B. 3D
• C. $-\frac{1}{4}D$
• D. -4 D

Answer 1

The correct option is C $-\frac{1}{4}D$

By using $f=\frac{xy}{x-y}\Rightarrow f=\frac{3\times 12}{3-12}=-4m$. Hence power $P=\frac{1}{f}=-\frac{1}{4}D$