Question

When two lenses 10 cm and -5cm are placed in contact then find the net power.

Answer 1

focal length of concave lens = -5 cm = -5 /100 m power = P₁ = 1/-5/ 100 = -20 D

focal length of convex lens = +10 cm = 10/100 m power = P₂ = 1/10/100 = 10D

So net power = P = P₁ + P₂ = -20D + 10D = -10 D (implies combination behave as convex lens).