Question

The magnification of an object is +2 when placed at 20 cm from a convex lens. To obtain magnification of –2, the object should be moved a distance equal to –

• A. 10 cm
• B. 20 cm
• C. 30 cm
• D. 40 cm

Answer 1

Answer: (B)

20 cm

Answer 2

When m = +2

Ž u = – x; v = – 2x; f = +20

$\frac { 1 } { \mathrm { v } }$ – $\frac { 1 } { \mathrm { u } }$ = $\frac { 1 } { \mathrm { f } }$

Ž – $\frac { 1 } { 2 \mathrm { x } }$ + = $\frac { 1 } { 20 }$

x = 10 cm

when m = – 2

u = – y; v = + 2y; f = +20

+ $\frac { 1 } { y }$ = $\frac { 1 } { 20 }$

Ž y = 30 cm

Ž y – x = 20 cm