Question

A real image of an object is formed at a distance of 20 cm from a lens. On putting another lens in contact with it, the image is shifted 10 cm towards the combination. The power of the lens is :

• A. 2 D
• B. 5 D
• C. 6 D
• D. 10 D

Answer 1

Answer: (B)

5 D

Answer 2

: As image formed is real, therefore lens must be convex, v = 20 cm. Let $\mathrm { f } _ { 1 }$ , be focal length for this lens.

$\therefore \frac { 1 } { \mathrm { f } _ { 1 } } = \frac { 1 } { \mathrm { v } } - \frac { 1 } { \mathrm { u } } = \frac { 1 } { 20 } - \frac { 1 } { \mathrm { u } }$

After placing it in contact with another lens, the image shifted to 10 cm towards the combination

i.e., v = (20 – 10) cm = 10 cm

$\Rightarrow \mathrm { f } _ { 2 } = 20 \mathrm {~cm}$