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?
(a). According to the given case, object is virtual, u=+12 cm The focal length of the convex lens is f=+20 cm. Using lens equation, v1−u1=f1 v1−121=201 v=7.5 cm (b). Now the focal length of the concave lens, f=−16 cm v1−u1=f1 v1−121=−161 v=48 cm
