Question

The distance of ral object when a concave mirror producers a real image of magnification ${}^{\prime }{m}^{\prime }$ is ($f$ is focal lenght)

• A. $\left(\frac{m-1}{m}\right)$
• B. $\left(\frac{m+1}{m}\right)$
• C. $(m-1)$
• D. $(m+1)$

Answer 1

Answer: (B)

$\left(\frac{m+1}{m}\right)$

Given, $m=-\frac{v}{u}$
$\Rightarrow v=-mu$
By mirror formula
$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$
$\frac{1}{f}=\frac{1}{-mu}+\frac{1}{u}$
$\Rightarrow \frac{1}{u}(-\frac{1}{m}+1)\frac{1}{f}$
or $u=\frac{f}{m}(m-1)$