Question

A square $ABCD$ of side $1mm$ is kept at distance $15cm$ infront of the concave mirror as show in the figure. The focal length of the mirror is $10cm$. The length of the perimeter of its image will be(nearly):

• A. $8mm$
• B. $2mm$
• C. $12mm$
• D. $6mm$

Answer 1

The correct option is A $8mm$

Given that for a concave mirror,

$f=-10cm$

$u=-15cm$

$heightofobject=0.1cm$

$h^{\prime }=?$

Using mirror formula

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

$\frac{1}{v}=\frac{-1}{30}$

$v=-30cm$

$magnificationm=\frac{h^{\prime }}{h}=\frac{-v}{u}$

$\frac{h^{\prime }}{0.1}=\frac{30}{-15}$

$h^{\prime }=-0.2cm$

So,

$PerimeterP=4\times 0.2$

$=0.8cm$

$=8mm$