Question

Image formed by a concave mirror focal length $6cm$, is $3$ times of the object, then the distance of object from mirror is

• A. $-4cm$
• B. $8cm$
• C. $6cm$
• D. $12cm$

Answer 1

The correct option is A $-4cm$

Given,

focal length, $f=6cm$

magnification, $m=3$

1) When magnification is positive: Image will be virtual

$m=3$

$f=-6cm$ (concave mirror)

We know,

$m=\frac{f}{f-u}$ ......(1)

$\Rightarrow 3=\frac{-6}{-6-u}$

$\Rightarrow u=-4cm$

2) When magnification is negative: image will be real

$m=-3$

$f=-6$

$\Rightarrow -3=\frac{-6}{-6-u}$ (from (1))

$\Rightarrow u=-8cm$

So, option A is correct