A converging lens of focal length `50cm` is placed co-axially in cintact with another lens of unknown focal length. If the combination behaves like a diverging lens of focal length `50cm`, find the power and nature of second lens.
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Here, `f_(1) = 50 cm`, `P_(1) = (100)/(f_(1)) = (100)/(50) = 2 D` `F = - 50 cm, P = (100)/(F) = (100)/(-50) = -2D` `:. P_(1) + P_(2) = P` `:. p_(2) = P - P_(1) = -2D - 2D = - 4D` As power is negative, lens must be concave.
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