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.

Here, f₁ = + 5.0 cm, and f₂ = –10.0 cm
Therefore, the combined focal length F is given by $\frac{1}{\mathrm{F}}=\frac{1}{{\mathrm{f}}_1}+\frac{1}{{\mathrm{f}}_2}=\frac{1}{5.0}-\frac{1}{10.0}=+\frac{1}{10.0}$
$\therefore$F = + 10.0 cm i.e., the combination behaves as a converging lens of focal length 10.0 cm.