Question

A convex lens of focal length f produces a virtual image n times the size of the object. Then the distance of the object from the lens is –

• A. (n –1) f
• B. (n + 1) f
• C. f
• D.

Answer 1

Answer: (C)

f

Answer 2

m =

so u = – u f = f m = + n

n = $\frac { f } { f + u }$ Ž f + u = $\frac { \mathrm { f } } { \mathrm { n } }$

u = – f =

u = – f $\left( 1 - \frac { 1 } { \mathrm { n } } \right)$ Ž u = – f $\left( \frac { \mathrm { n } - 1 } { \mathrm { n } } \right)$

so |u| = $\left( \frac { \mathrm { n } - 1 } { \mathrm { n } } \right)$ f