Question

The magnification of an object placed in front of a convex lens of focal length $20cm$ is $+2$. The image distance is________.

• A. $10cm$
• B. $20cm$
• C. $-30cm$
• D. $-20cm$

Answer 1

The correct option is D $-20cm$

Given:
Magnification, $m=\frac{v}{u}=+2$
$\therefore$ Object distance $u=-x$
Image distance $v=-2x$
Focal length $f=+20cm$

Using Lens formula $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\frac{1}{-2x}-\frac{1}{-x}=\frac{1}{20}$
or $x=10cm$
Hence,
Object distance, $u=-10cm$
Image distance, $v=-20cm$