A convex lens forms inverted image of a real object on a fixed screen. The size of image is 9 cm . When lens is displaced 40 cm along principle axis it again forms a real image of size 4 cm on the screen. Focal length of the lens is .

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Correct answer: A. 48 cm

Since m 1 m 2 = 1 ⇒ h 1 h 0 h 2 h 0 = 1 h 0 = h 1 h 2 = 6 c m for first position of the lens m = - 3 2 = - x + 40 x ⇒ 3 x = 2 x + 80 ⇒ x = 80 u = - x , y = ( 40 + x ) 1 f = 1 120 - 1 - 80 ⇒ f = 48 cm

A convex lens forms inverted image of a real object on a fixed screen. The size of image is 9 cm . When lens is displaced 40 cm along principle axis it again forms a real image of size 4 cm on the screen. Focal length of the lens is .

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