An illuminated object and a screen are placed `90 cm` apart. What is the focal length and nature of the lens required to produce a clear image on the screen twice the size of the object ?
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As image is real, the lens must be convex and it should be placed between the object and screen. Let `x` be distance between the object and convex lens. `:. u = - x, v = 90 - x , m = - 2` As `m = (v)/(u)`, `:. -2 = (90 - x)/(- x), x = 30 cm` `:. u = - x = - 30cm`, `v = 90 - x = 90 - 30 = 60 cm` `(1)/(f) = (1)/(v) - (1)/(u) = (1)/(60) - (1)/(-30) = (3)/(60) = (1)/(20)` `f = 20 cm`
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