Question

A point object is placed at distance of 20 cm from a lens as shown in figure. The position of final image from the lens is

Answer 1

In the given figure ,

For plane mirror the virtual image act as virtual obj and forms image at $70cm$ from mirror

$\begin{array}{c}\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\\ \frac{1}{v}-\frac{1}{-20}=\frac{1}{30}\\ \frac{1}{v}=\frac{2}{60}-\frac{3}{60}\\ \frac{1}{v}=-\frac{1}{60}\end{array}$

for lens real image acts as object for lens

$\begin{array}{c}\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\\ \frac{1}{v}-\frac{1}{-80}=\frac{1}{30}\\ \frac{1}{v}=\frac{1}{30}-\frac{1}{80}\\ \frac{1}{v}=\frac{8}{240}-\frac{3}{240}\\ \frac{1}{v}=\frac{5}{240}\\ v=48cm\end{array}$