Question

The image of the needle placed 45 cm from a lens is formed on the other side of the lens. The displacement of the image, if the needle is moved by 5.0 cm away from the lens is

• A. 10 cm, toward the lens
• B. 15 cm, toward the lens
• C. 15 cm, toward the lens
• D. 10 cm, toward the lens

Answer 1

Answer: (C)

15 cm, toward the lens

Answer 2

: Here, $u = - 45 \mathrm {~cm} , \mathrm { v } = 90 \mathrm {~cm}$

$\Rightarrow \mathrm { f } = 30 \mathrm {~cm}$

when the needle is moved 5 cm away from the lens,

$\therefore \frac { 1 } { \mathrm { v } ^ { \prime } } = \frac { 1 } { \mathrm { f } } + \frac { 1 } { \mathrm { u } ^ { \prime } } = \frac { 1 } { 30 } + \frac { 1 } { - 50 } = \frac { 2 } { 150 }$

$\therefore$ Displacemetnt of image towards the lens.