Question

Two lenses of powers $+15D$ and $-5D$ are in contact with each other. What is the focal length of combination?

Answer 1

Given : $P_1=+15D$ $P_2=-5D$

Power of combination $P_{eq}=P_1+P_2=15-5=10D$

Focal length of combination $F_{eq}=\frac{1}{P_{eq}}=\frac{1}{10}=0.1m=10cm$