1. The problem statement, all variables and given/known data Consider a special crystal sphere, where light from a distant object is focused by the front surface of the sphere exactly onto the opposite side of the sphere. What is the index of refraction of the sphere? Assume the sphere is in air. 2. Relevant equations (n'/l') - (n/l) = (1/R)*(n' - n) f' = n'/K = n'F 3. The attempt at a solution I know that n is taken to be 1 since the sphere is in air. Since the object is described to be distant, I'm guessing that perhaps l could be infinity. I've thought about trying to relate different angles to each other or looking at the sag, but it looks like the question is asking for a numerical value, and I don't know how to get there with what was given. Because it's focusing at the side of the sphere, we may be able to relate the radius to the focal point?