Question

a convex lens of focal length 40 cm is placed in a contact with a concave lens of focal length 25 cm what is the power of the combination​

Answer 1

Convex lens f = +40 cm  as the principal focal point is in the direction of the rays.

concave lens = -50 cm as the principal focal point is on the side of the object.

combination of thin lenses:  1/f = 1/f1 + 1/f2 = 1/40 - 1/50 = 1/200 

    f = 200 cm  positive so convex lens

P1 = 1 /(40/100) meter = 2.5 Dioptre

P2 = 1/(-50/100)m = -2 D 

power of combination = P = P1 + P2 = 2.5 - 2.0 = 0.5 D

                or,  1/(200/100)m   = 1/2 = 0.5 D

Answer 2

focal length of convex lens is 40 and that of concave is - 25 and they both are in contact so use the formula 1/f = 1/f1 + 1/f2 so here u will get the total focal length then we know the formula of power = 1/f...so just put it in the formula ....and plzz be careful about the SI unit...take SI unit os focal length in metre so u can get the ans in dioptre