Question

A convex lens of focal length 10cm is placed in closed contact with a concave lens of focal length 5cm. The power of combination is
20 D
10 D
-20 D
-10 D​

Answer 1

Answer:

Option d) -10D is correct

Explanation:

Given :

Focal length of convex lens  f₁ = + 10 cm = + 0.1 m

Focal length of concave lens f₂ = - 5 cm = - 0.05 m

Power of convex lens P₁ = 1/f₁ = + 1/0.1 = + 10 D

Power of concave lens P₂ = 1/f₂ = - 1/0.05 = - 20 D

Power of combination P = P₁ + P₂

                                        = 10 - 20

                                        = - 10 D