Question

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?
​

Answer 1

Answer:

Here , the point P on the right of the lens acts as a virtual object

u=12 cm , v=?

(a) f=20cm

As  

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

As,  

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.

Hope this helps!