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. – 3/4 D
• B. 3 D
• C. – 1/4 D
• D. – 4 D

Answer 1

Answer: (C)

– 1/4 D

Answer 2

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