Question

A convex lens of power 5 D and a concave lens of power 7.5 D are placed in contact with each other. What is the:
(a) Power of this combination of lenses?
(b) Focal length of this combination of lenses ?

Answer 1

(a)
Power of a convex lens is positive, and that of a concave lens is negative.
Power of convex lens, $P_1=+5D$
Power of concave lens, $P_2=-7.5D$

Power of a combination of lens is equal to the sum of the individual powers. So, power of the combination will be:
$P=+5+(-7.5)$
$P=-2.5D$

(b)
Focal length is equal to the inverse of power. So, the focal length of the combination is:
$f=\frac{1}{P}$
$f=\frac{1}{-2.5}=-0.4m$