Question

Two lenses one of focal length 20 cm (convex lens) and another of focal
length 15 cm (concave lens) are placed in contact. What is the focal
length and power of the combination?​

Answer 1

Given:

F of convex mirror=20cm

F of concave mirror=-15cm

Total focal length=

$\frac{1}{f} + \frac{1}{f} = p$

As this in cm

100/20+(-100/15)=5-20/3

=-5/3

p=-1.66D

Focal length of combination= -3/5×100(because its in m and f is reciprocal of power)

F=(-60cm)

Hope it helps in your exams!