Question

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?

Answer 1

Here the point p on the right of the lens as a virtual image

U=12cm,v=?

(a) f=-20cm

1/f =1/u-1/v

1/v-1/12=1/20

1/v=1/20+1/12=8/60

V=60/8=7.5cm.

(b) f=-16cm, u=12cm

1/f =1/v-1/u

1/v=1/f+1/u

1/v=-1/16+1/12

=1/48

V=48cm