This is a two part question. First question: what exactly is a photon? I know that it's essentially a bundle of energy, but does it have mass? Can it be related by E=mC^2? Second question: If I were to find a perfectly smooth spherical mirror (also assuming the distance between atoms is sufficiently small that the light cannot escape) and somehow turn on a light inside, would the photons somehow build up and become jammed in there?