Question

A concave mirror of focal length $20cm$ is placed $50cm$ from a wall. How far from the wall an object be placed to form its real image on the wall? $(-\frac{100}{3}cm)$

Answer 1

Given,

Focal length $f=20cm$

Image distance, $v=50cm$

from mirror formula,

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

$\Rightarrow$ $\frac{1}{-20}=-\frac{1}{50}+\frac{1}{u}$

$\frac{1}{u}=\frac{1}{50}-\frac{1}{20}$

$=\frac{20-50}{50\times 20}$

$=\frac{-30}{1000}$

$u=-\frac{100}{3}cm$