A double convex lens is made of a glass of refractive index 1.55, with both faces of the same radius of curvature. Find the radius of curvature required, if the focal length is 20cm.

Double convex lenses are to be manufactured with both a glass of same refractive index and same radius of curvature.
Given μ=1.55 and one glass radius be R1=R while other be R2=-R
Focal length f=20cm
Now 1/f = (μ-1)[1/R1 - 1/R2] = (μ-1)[1/R + 1/R] = (μ-1)(2/R)
So, 1/20 = (1.55-1)(2/R)
1/20 = 0.55(2/R)
R = 20 * 0.55 * 2 = 22cm