Question

The focal length of a plano-convex glass lens is 20cm muo of glass =1.5. The plane face of it is silvered. An illuminating object is placed at a distance of 60cm from the lens on its axis along the convex side. Then the distance of the image in cm is

Answer 1

Plane surface of plane – convex lens is silvered and it then acts like a concave mirror of focal length:

1/F=2/f

F=f/2=20/2=10 cm.

1/F=1/o+1/i

1/10=1/60+1/i

i=12 cm.