# 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 cmThe focal length of the convex lens is

f=+20 cm.

Using lens equation, 1v−1u=1f

1v−112=120

v=7.5 cm

(b). Now the focal length of the concave lens, f=−16 cm

1v−1u=1f

1v−112=1−16

v=48 cm