Question

The distance between an object and the screen is 100 cm. A lens produces an image on the screen when placed at either of the positions 40 cm apart. The power of the lens is

• A. 3 D
• B. 5 D
• C. 7 D
• D. 9 D

Answer 1

Answer: (B)

5 D

Answer 2

By using $f = \frac { D ^ { 2 } - x ^ { 2 } } { 4 D }$ ⇒ $f = \frac { 100 ^ { 2 } - 40 ^ { 2 } } { 4 \times 100 } = 21 \mathrm {~cm}$

Hence power $P = \frac { 100 } { F ( \mathrm {~cm} ) } = \frac { 100 } { 21 } \approx + 5 D$