Question

An object 1.00cm high is placed 18.0cm from a converging lens forms a real image 2.00cm high. Calculate the focal length of the lens

Answer 1

$$m=-\frac{v}{u}=-\frac{v}{18}=-\frac{2}{1}\\v=36\ cm\\ \frac{1}{f}=\frac{1}{18}+\frac{1}{36}\\f=12\ cm$$