The distance between object and its two times magnified real image as produced by a convex lens is 45 cm . The focal length of the lens used is ________ cm .

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Magnification, m = v u = - 2 ⇒ v = - 2 u … i As, v - u = 45 … ii ⇒ u = - 15 cm Then, v = 30 cm From lens equation, we can write 1 f = 1 v - 1 u ⇒ 1 f = 1 30 - 1 - 15 ⇒ f = + 10 cm

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