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

Given

f₁ = 25cm (Focal length of convex lens is positive)

⇒ f₁ = 0.25m (1m = 100cm)

P (in dioptre) =

⇒ Power of convex lens,(F=25 cm) P₁ =

⇒ P₁ =
∴ P₁ = 4D

f₂ = -10cm (Focal length of concave lens is negative)

⇒ f₂ = -0.1m (1m = 100cm)

⇒ Power of the concave lens(F=10 cm), P₂ =

⇒ P2 =

∴ P2 = -10D.

Power of combination = P₁ + P₂.

P = 4D – 10D = -6D.

Lens power of the combination is -6D. Since it is negative therefore the new lens will behave like concave lens having a focal length of 16.6cm.