For example, when you have a photon interacting with a bound electron, there is a work function relating the minimum energy photon capable of interaction (given by hf). My question is, what exactly is going on in this situation if the work function is exactly equivalent or less than the energy of the incident photon? How exactly does the photon and electron interact? Also, is there anything similar to this? For example, an electron hitting a proton must have a particular energy requirement for interaction (right?). What exactly determines the threshold frequency necessary for interaction with a proton (is it the nuclear forces)? How about for structures composed of more than one atomic entity (e.g. sugar molecule)? Can a photon interact with the molecule as a whole or must it interact with distinct components (e.g. electrons, protons)? Can essentially anything interact with something else if it has enough energy? I guess my question boils down to: do all things exhibit a "threshold frequency"? Please let me know if you need any further clarifications or if I'm grossly mistaken on any points.