A convex lens of focal length 2S cm and a concave lens of focal length 10 cm are placed in close contact with one another.

(a) What is the power of this combination?


(b) What is the focal length of this combination?


(c) Is this combination converging or diverging ?

Focal length of convex lens= 25 cm = 0.25 m


Power = 1/f = 1/0.25 = + 4 D


Focal length of concave lens = -10 cm = -0.1 m


Power = 1/f = -1/0.1 = - 10 D


(a) Power of combination = P1+ P2 = 4 D – 10 D = -6 D


(b) f = 1/ P = 1/ -6 = -16.66 cm


(c) This combination is Diverging


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