Until now I've learned that light is electromagnetic waves. I haven't learned about photons yet, but I'm guessing that photons are waves according to the electromagnetism theory (or quantum mechanics?, or both?). My question is "when do you consider photons as particles"? Particles in the sense of particles in classical mechanics I think I'm meaning. Is my question too vague? I've heard about a duality wave/particle of photons, but I've no idea when photons can be considered as particles and if they are particles under certain circumstances. Thanks for any enlightenment.