What is the focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm? is the system a converging or a diverging lens ? Ignore thickness of the lenses.

Explanation

`f_(1) = 30 cm , f_(2) = - 20 cm `
f = `(f_(1)f_(2))/(f_(1) + f_(2)) = (30 xx -20)/(30 - 20) = (-600)/(10) = - 60 ` cm
.f. being negative, the combination is a diverging lens.