Question

a convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 25 centimetre find the power of combination of the lens in dioptre ​

Answer 1

Given

focal length of convex lens f₁ = 20 cm and

focal length of concave lens f₂ = - 25 cm (according to sign convention)

Thus the combined focal length of the lens will be f = f₁ + f₂ = 20 -25 = - 5 cm

And we know power of the lens P = (1/f) = (1/-5) = - 0.2 D