Question

Two lenses of power $10D$ and $-10D$ are in contact, find power, focal length and nature of lens combination.

Answer 1

Given : $P_1=10D$ and $P_2=-10D$
Power of combination of lenses $P_{net}=P_1+P_2$
$\therefore$ $P_{net}=10-10=0D$
Focal length of combination of lenses $F_{net}=\frac{1}{P_{net}}=\frac{1}{0}=\infty$
Since focal length of plane lens is infinite. So above combination of lens becomes a plane lens.