I'm currently self-studying quantum mechanics and instead of starting a new thread every time I have a new question I figure I'd just make one thread dedicated to all of them. I'm going over the Photoelectric Effect. The way I understand it is when light is shone on a metallic surface, the photons provide energy to free electrons from the surface. Since this is instantaneous and varies with frequency (not intensity), this shows evidence that light behaves as a stream of particles as opposed to a wave. First, correct me if my understanding is wrong. Second, here is my question: so if the electrons are freed from the energy provided by the photon, where exactly does this energy come from and what does this say about the photon? At first it appears to come from conservation of momentum (KE is exchanged from the photon to the e- with some of the energy absorbed into the surface), but clearly this must be impossible since photons are massless. Does this imply that photons are essentially just packets of energy? How does this reaction occur?