Question

If the distance between object and its image is 120cm, find focal length of mirror. Image of object is behind mirror, erect and length is 3 times larger than object.

• A. 30cm
• B. 40cm
• C. 45cm
• D. 55cm

Answer 1

The correct option is C 45cm

Since,

Magnification $=0$

Height of image / Height of object $=3=$ Distance of image / Distance of object

$\left(DI\right)$ $=3\left(DO\right)$

$x=3(120-x)$

$x=90cm$

$DO=30cm$

Now,

$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$

$\frac{1}{f}=\frac{1}{30}-\frac{1}{90}$

$f=45cm$