A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close focal contact with each other. Calculate the lens power of this combination.

Question

Philoid · Accepted Answer

Given;

Focal Length of convex lens f₁ = 25cm = 0.25m (1m = 100cm);

Focal Length of concave lens f₂ = -10cm = -0.1m (1m = 100cm);

Power of convex lens P₁ =

∴ P₁ = 4D.

Power of concave lens P₂ =

∴ P₂ = -10D.

Power of combination = P₁ + P₂ = 4-10 = 6D.

Thus, power of combination is -6 Dioptre.