Question

Two lenses have focal lengths +10cm and -15cm. They are kept in contact. What is the focal length of combined lens? What is the nature of the lens and also find the power of the combined lens.

Answer 1

Convex lens of f = +10cm or
= +10/100 m = +0.1m
Concave lens of f = -15cm or
= -15/100m = -0.15m
( nature of lenses )
so, combined F = ( +0.1 - 0.15 )m
= -0.05m

power of convex lens = 1/+0.1 = 10 D

power of concave lens = 1/-0.15 = -0.67 D
Now,
combined power of lenses = (+10 - 0.67)D
= +9.33D