A combination of lenses for a camera contains two converging lenses of focal lengths 20 cm and 40 cm and a diverging lens of focal length 50 cm. Find the power and focal length of the combination.

Focal length of 1 lens = 20 cm = 0.2 m

Power 1 = 1/ 0.2 = +5 D


Focal length of 2nd lens = 40 cm = 0.4 m


Power 2 = 1/ 0.4 = 0.25 D


Power = P1 + P2 = +5D + 0.25 D = + 5.25 D ;


P = 1/f


f = + 18.18 cm


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