Fri March 02, 2012 By: Debasish Panigrahi

A beam of light coverges at point p. now a lens is placed in the path of convergent beam 12cm from p. at what point does the beam converges if the lens is (a) convex lens of focal length 20cm. (b) A CONCANVE LENS OF FOCAL LENGTH 16CM. EXPLAIN WITH RAY DIAGRAM FOR BOTH THE CASES

Expert Reply
Sun March 04, 2012
Here , the point P on the right of the lens acts as a virtual object
u=12 cm , v=?
(a) f=20cm

1/f =1/v -1/u

1/v -1/12= 1/20

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

v=60/8= 7.5cm

Image is at 7.5 cm to the right of the lens where the beam converges.

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


1/f =1/v -1/u

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

      = 1/48

v= 48cm

Hence image is at 48cm to the right of the lens, where the beam would converge.

Ask the Expert