Q: A converging lens of focal length 5.0 cm is placed in contact with a diverging lens of focal length 10.0 cm Find the combined focal length of the system.

Sol: Here f_{1} = + 5.0 cm and f_{2} = -10.0 cm

Therefore, the combined focal length F is given

$\large \frac{1}{F} = \frac{1}{f_1} + \frac{1}{f_2} $

$\large \frac{1}{F} = \frac{1}{5} – \frac{1}{10} = \frac{1}{10}$

F = + 10.0 cm ie. the combination behaves as a converging lens of focal length 10.0 cm.