Question

A convex lens of focal length 80 cm and a concave lens of focal length 50 cm are combined together. What will be their resulting power?(a) + 6.5 D(b) –6.5 D(c) + 7.5 D(d) –0.75 D

Answer 1

given,

f1 = 80 cm = +0.8m

f2 = -50 cm = -0.5m

let the resulting power be P.

therefore,

P1 = 1/f1 = 1/0.8 = +1.25 D

and

P2 = 1/f2 =1/0.5 = -2D

since,

P = P 1 + P2

P = + 1.25 + (-2) D

P = -0.75 D .

hence,

option (d) -- 0.75 D is correct