# A convex lens of focal length 40 cm and a concave lens of focal lenght 50 cm are placed in contact with each other.(i) calculate the power of lens (ii) the focal lenght of the lens combination

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by vishagh

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by vishagh

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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

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

concave lens = -50 cm

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