The image obtained with a convex lens is erect and its length is `4` times the length of the object. If focal length of the lens is `20cm`, calculate the object and image distances.
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As image formed is erect and hence virtual, the magnification produced by the lens is positive, i.e., `m = + 4` `f= + 20 cm, u = ?` `4 = (20)/(u + 20), 4u + 80 = 20` `u = (20 - 80)/(4) = - 15 cm` Again, from `m = (f - v)/(f)` `4 = (20 -v)/(20)` `80 = 20 - v` `v = 20 - 80 = - 60 cm`
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