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

Explanation

Here `f_1 = + 5.0 cm f_2 =- 10.0 cm`
therefore the combined focal length F is given by
` (1)/(F) = (1)/(f_1 )+(1)/(f_2) =(1)/( 5.0 ) -(1)/(10.0 ) = + (1)/( 10.0)`
` therefore F = + 10.0 cm ` i.e., the combination behaes as a converging lens of focal length 10.0 cm