1. The problem statement, all variables and given/known data We are to derive the Gaussian formula for a case with a virtual object, a virtual image, and a concave surface. 2. Relevant equations Snell's law, n(sin(phi)) = n'(sin(phi')) geometry 3. The attempt at a solution I can do a derivation for a situation with a point object placed to the left of a convex surface with a real image being formed, but I don't understand what it means to have a virtual object together with a virtual image, much less for a concave surface.