Two lenses of power -3.5 D and + 1 D are placed in contact. Find the total power of the combination of lens. Calculate the focal length of this combination.

Question

Philoid · Accepted Answer

Power P1 = -3.5D; P2 = +1D

Total power = P1 + P2 = -3.5D + 1D

⇒ Total power (P) = -2.5D.

Now;

P (in diopter) =

⇒ -2.5 =

⇒

Now 1m = 100cm

⇒ -0.4m = -40cm

Hence the focal length of lens is -40cm.