Question

7. A convex lens of focal length 40 cm, a concave lens
of focal length 40 cm and a concave lens of focal
length 15 cm are placed in contact. The power of
this combination in dioptre is
(a) + 1.5
(b) - 1.5
(C) + 6.67
(d) - 6.67​

Answer 1

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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