Keep what waves explain
Wave optics describes propagation and interference successfully. Einstein asks whether energy exchange with matter needs an additional, explicitly heuristic description.
Read · Light quanta: from entropy to an energy scale · Results
Introduction · a heuristic viewpoint
Einstein argues that light, when it is emitted or absorbed, behaves as if its energy came in separate quanta, and calls the idea a heuristic point of view. Follow all nine sections, from the trouble with classical radiation to the photoelectric effect.