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

In the given situation, the object is virtual and the image formed is real. Object distance, u = +12 cm 

(a) Focal length of the convex lens, f = 20 cm 

Image distance = v 

According to the lens formula, we have the relation:

Hence, the image is formed 7.5 cm away from the lens, toward its right. 

(b) Focal length of the concave lens, f = −16 cm 

Image distance = v 

According to the lens formula, we have the relation:

Hence, the image is formed 48 cm away from the lens, toward its right.