Question

A rod of length $10\,cm$ lies along the principal axis of a concave mirror of focal length $10\,cm$ in such a way that its end closer to the pole is $20\,cm$ away from the mirror. The length of the image is,

• A. $2.5\,cm$
• B. $5\,cm$
• C. $10\,cm$
• D. $15\,cm$

Answer 1

Answer: (B)

from mirror formula $\frac{1}{v}+\frac{1}{u}=\frac{1}{f}$ image distance of $A$
$\frac{1}{v_{A}}+\frac{1}{\left(- 30\right)}=\frac{1}{- 10}\Rightarrow v_{A}=-15\,cm$
Also image distance of $C,v_{c}=-20\,cm$
The length of image
$=\left|v_{A} - v_{c}\right|=\left|\right.-15-\left(\right.-20\left.\right)\left|\right.=5\,cm$