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An illuminated object and a screen are placed 90 cm apart. Determine 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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Answer» Solution :It is GIVEN that distance between an illuminated object and screen `|u+v| = 90 cm` and linear magnification of image m = 2 Since the image is being formed on the screen, it is a real image hence the lens used is a convex lens. `therefore |m| =|v/u| =2` or `|v|=2|u| rArr |u|=30 cm` and `|v| = 60 cm` As PER SIGN convention followed u is -ve and v is +ve. Hence, using lens formula `1/v - 1/u = 1/f`, we have `1/(+60) -1/(--30) = 1/f rArr 1/f = 1/60 + 1/30 = 1/20` or `f=+20` cm |
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