Any help much appreciated - this is driving me crazy! 1. The problem statement, all variables and given/known data Show that the focal length of a half ball lens, in air, (in the paraxial limit) is given by: f=R/n-1 2. Relevant equations Equation (from Hecht- Optics), for refraction at a spherical interface: nm/s0 + nl/s1 = (nl-nm)/R where, nm = refractive index of medium nl = refractive index of lens material s0 = distance from object to surface s1 = distance from image to surface 3. The attempt at a solution Setting s0 = ∞ Then s1 = f Subbing into the equation gives, nm/s0 + nl/s1 = (nl-nm)/R 0 + nl/f = (nl-nm)/R f = Rnl/nl-1 Where am I going wrong with this? Thanks.