Question

An object is placed at $20\,cm$ in front of a concave mirror produces three times magnified real image. What is focal length of the concave mirror ?

• A. 15 cm
• B. 6.6 cm
• C. 10 cm
• D. 7.5 cm

Answer 1

Answer: (A)

15 cm

Answer 2

We know that
Linear magnification $m=\frac{f}{f-u}$
Given, object displaced $u=-20 \,cm$
$m=-3$
$(\because$ all real image are inverted)
So, $-3=\frac{f}{f-(-20)}$
$-3=\frac{f}{f+20}$
$\Rightarrow-3 f-60=f$
$4 f=-60$
$\Rightarrow f=-\frac{60}{4}=-15 cm$
Since, mirror is concave $f=-15 \,cm$.