I'm working through some questions on lenses, and i'm a bit stuck on this one involving radius of curvature. The part of the question I'm having difficulty with is The lens is made of crown glass of refractive index n=1.51. The radii of both surfaces is the same. What is the radius of curvature of the surfaces? The lens is bi-convex ([tex]f=+20cm[/tex]). The first part of the question involves finding heights, natures and magnifications of objects which is fine, but this part doesn't make any sense. I thought about using the Lens' Maker's equation but if [tex]R_1=R_2[/tex] would the right-hand-side of this equation not equal zero?