Question

A person's near point is 50 cm and his far point is 3 m. Power of the lenses he requires for

(i) Reading and

(ii) For seeing distant stars are

• A. – 2 D and 0.33 D
• B. 2 D and – 0.33 D
• C. – 2 D and 3 D
• D. 2 D and – 3 D

Answer 1

Answer: (B)

2 D and – 0.33 D

Answer 2

For correcting the near point, required focal length

$f = \frac { 50 \times 25 } { ( 50 - 25 ) } = 50 \mathrm {~cm}$

So power $P = \frac { 100 } { 50 } = + 2 D$

For correcting the far point, required focal length

$f = - ($ defected far point $) = - 3 m$