Question

The magnification of an object placed in front of a convex lens is $+2$. The focal length of the lens is $2.0$ metres. Find the distance by which object has to be moved to obtain a magnification of $-2$ (in cm).

Answer 1

For magnification $+2,u=-x,v=-2x$ and $f=2.0m$
From $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$, we have $\frac{1}{-2x}+\frac{1}{x}=\frac{1}{z}$
$x=1.0m$
For magnification of $-2,u=-y,v=+2y,f=2.0m$
we have $\frac{1}{2y}+\frac{1}{y}=\frac{1}{2}\Rightarrow y=3.0cm$.

So, object has to be moved by

3cm - 1 cm =2 cm