Question

A lens of focal lengths $20cm$ produces a real image of a stationary object having magnification $2$. Find the distance by which the lens should be moved so that it produces a virtual image of the same magnification.

• A. $10cm$
• B. $20cm$
• C. $15cm$
• D. $40cm$

Answer 1

The correct option is B $20cm$

Magnification$=\frac{f}{f-d_0}$ where;

$f=$focal length of lens and $d_0=$ distance between lens and object

Since the lens is producing a real image, $M$ is negative

$-2=\frac{20}{20-d_0}$

Thus, $d_0=30cm$

For virtual image, the magnification is positive

$2=\frac{20}{20-d_0}$

Thus, $d_0=10cm$

Hence, lens must be moved by $30-10=20cm$