Q. 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 20cm, and (b)a concave lens of focal length 16 cm?

Answer

In the given circumstances the object is virtual and the image formed is real.

A Object distance u=+ 12cm

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

Image distance=v

As-per to the lens formula we have the relation:

As a result the image is formed7.5cmawayas of the lens toward its right.

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

Image distance=v

As-per to the lens formula we have the relation:

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