A beam of light converges at a point P. Now a lens is placed in the path of the convergent beam 12 cm from P. At what point does the beam converge if the lens is (a) a convex lens of focal length 20 cm, and (b) a concave lens of focal length 16 cm?
(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, 1v−1u=1f
(b). Now the focal length of the concave lens, f=−16 cm