1. The problem statement, all variables and given/known data Its not a specific question but a general one to solve similar questions. I need to find focal length of a lens silvered at one face 2. Relevant equations 3. The attempt at a solution I know a convex lens silvered at one face will behave as an effective concave lens. But i need to find the equations. I can do it by individually solving each case but need a faster one. My book says that the effective power, P = P(L) + P(M) +P(L).....P(L) is power of lens P(M) is power of mirror. I dont know why this works? For two lenses kept close i understand why P = P1 + P2 but don't how this works in lens-mirror case too. Any Help?