# A concave lens has focal length of 20 cm. At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the

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A concave lens has focal length of 20 cm. At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens? Also, calculate the size of the image formed.

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Here, f = -20 cm
u =?, h_(1)=5cm v = -15cm, h_(2)=?
As (1)/(v) - 1/u = (1)/(f) :. 1/u = (1)/(v) - (1)/(f) = 1/-15 + 1/20 =(-1)/(60)
u = -60cm
From m=(h_(2))/(h_(1))=v/u, (h_(2))/(5) = (-15)/(-16)=1/4, h_(2) = 5/4cm =1.25cm
This is the height of virtual image.