Question

A convex lens of focal length 40 cm and a concave lens of focal length 25 cm are placed in contact in such a way that they have the common principal axis. Find the power of the combination.

Answer 1

Focal length of the convex lens, $f_1$ = 40 cm = + 0.4 m

Power of the convex lens,

$P_1$ = $\frac{1}{+0.40}$ = + 2.5 D

Focal length of the concave lens,

$P_2$ = $\frac{1}{-0.25}$ = - 4 D

Power of the combination,

P = $P_1$ + $P_2$ = +2.5 D - 4 D = - 1.5 D