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A beam of light converges at a point P. ...

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

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Here the object is virtual and the image is real. `u = +12cm` (object on right, virtual)
(a) `f = +20cm`. Image is real and at 7.5 cm from the lens on its right side.
(b) `f = -16 cm`. Image is real and at 48cm from the lens on its right side.
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