Q. Two thin lenses, one of focal length $+ 60\, cm$ and the other of focal length $- 20\, cm$ are put in contact, the combined focal length is
AMUAMU 2004
Solution:
The focal length $(f)$ of the combination of concave and convex lens is
$\frac{1}{f}=\frac{1}{f_{1}}+\frac{1}{f_{2}}$
where $f_{1}=+60\, cm$,
$f_{2}=-20\, cm$
$\therefore \frac{1}{f}=\frac{1}{60}-\frac{1}{20}$
$\Rightarrow \frac{1}{f}=\frac{-2}{60}$
$\Rightarrow f=-30\, cm$
