Question

Two thin lenses of focal length f₁ and f₂ are coaxially placed in contact with each other then the power of combination is ________ .

(A) [(f₁ + f₂) / 2]

(B) √(f₁ / f₂)

(C) [(f₁f₂) / (f₁ + f₂)]

(D) [(f₁ + f₂) / (f₁f₂)]

Answer 1

The correct option is (D) [(f₁ + f₂) / (f₁f₂)].

Explanation:

 As lances are in contact

(1/f) = (1 / f₁) + (1 / f₂)

(1/f) = [(f₁ + f₂) / (f₁f₂)]

Power = {1 / (focal length)} = (1/f) = [(f₁ + f₂) / (f₁f₂)]