A screen is placed 90 cm from an object. The image an object on the screen is formed by a convex lens two different locations separated by 20 cm. the focus length of the lense is
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Here, distance between object and screen, `D = 90 cm` Distance between two locations of convex lens, `d = 20 cm, f = ?` As `f = (D^(2) - d^(2))/(4 D) :. F =(90^(2) - 20^(2))/(4 xx 90) = ((90 + 20) (90 - 20))/(360) = (110 xx 7)/(36) = 21.4 cm`
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