Refraction at a single surface

[@sammyapsel1443]

why is it that OPL is constant if we want to focus all rays in one point?

[@warlocksparks]

by fermat's principle we know the possible paths are local minima of OPL. If we want the light to end up at the same focus point no matter where it goes through the lens then all paths through the lens to the focus point have to be the same OPL (part of the same local minimum). otherwise there is only one local minimum path that reaches the focus point and every divergence from that path will result in light from the source not passing through focus