Question

A plano convex lens of refractive index 1.5 and radius of curvature 30cm is silvered at the curved surface. Now this lens has been used to form the image of an object. At what distance from this lens an object be placed in order to have a real image of the size of the object?

(a) 20cm 

(b) 30cm 

(c) 60cm 

(d) 80cm.

Answer 1

(a) : A planoconvex lens behaves like a concave mirror when its curved surface is silvered.

Therefore, F of concave mirror so formed

= R/μ2 = 30/(2 x 1.5) =10cm

To form an image of object size, the object should be placed at (2F) of the concave mirror.
Therefore, Distance of object from lens = 2 × F
= 2 × 10 = 20cm.