Question

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

Answer 1

Given focal length of the convex lens f₁ = 20 cm

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

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

Power of the lens P = (1/f) = 1/-5 = 0.2 D