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,

Options

A. 15 cm
B. `-15 cm`
C. `-30 cm`
D. 30 cm

Answer

C

Explanation

`F=(f_(1)f_(2))/(f_(1)+f_(2))=(60xx(-20))/(60-20)=-30 cm`