two lenses of focal length 20 cm and 40 cm are kept in contact with each other the power of equivalent length will be
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f1 = 20cm = 20 cm × 1 m / 100 = 1/5 meter
f2 = 40cm = 40 cm × 1m / 100 = 2 /5 meter
p1 = 1 / f1 = 1 / ( 1 / 5 ) = 5 D
p2 = 1 / f2 = 1 / ( 2 / 5 ) = 5 / 2 D = 2.5 D
equivalent power , p = p1 + p2 = 5 + 2.5 = 7.5 D
f2 = 40cm = 40 cm × 1m / 100 = 2 /5 meter
p1 = 1 / f1 = 1 / ( 1 / 5 ) = 5 D
p2 = 1 / f2 = 1 / ( 2 / 5 ) = 5 / 2 D = 2.5 D
equivalent power , p = p1 + p2 = 5 + 2.5 = 7.5 D
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