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A beam of light converges to a point P ,...

A beam of light converges to a point P , A lens is placed in the path of the convergent beams 12 cm from P. At what point does the beam converge if the lens is
a. A convex lens of focal length 20 cm.
b. A concave lens of focal length 16 cm ?

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The correct Answer is:
7.5 cm b. 48 cm
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