Question

The focal length of a double convex lens is equal to radius of curvature of either surface The refractive index of its material is

Answer 1

Focal length of a lens is given by:

1/f = (u-1) (1/r1 + 1/r2)

by the iven information we have got
r1 = r2= f = r

1/r = (u -1) (1/r + 1/r)

or

u = 1/2 + 1
= 3/2
= 1.5
(u=refractive index)