The power of a concave lens is 10 D and that of a convex lens is 6 D. When these two lenses are placed in contact with each other, the power of their combination will be :(a) +16 D (b) +4 D (c) −16 D (d) −4 D

(a) -4 D

Explanation

Given:

Power of the concave lens, $P_1$ = $-$10 D   (power of concave lens is always taken negative)
Power of the convex lens, $P_2$ = $+$10 D    (power of convex lens is always taken positive)

To find: Combined power of the two lenses, $P$.

Solution:

The power of the combination of lenses is given as-

$P=P_1+P_2$

Substituting the given value, we get-

$P=6D+(-10D)$