We extend our previous work on scalar quantum particle production by moving mirrors in two-dimensional flat space-time to models with asymptotically null trajectories. This proves to have considerable heuristic value in understanding the mechanism of quantum particle emission from black holes. We demonstrate that Hawking's derivation of that phenomenon using ray-tracing is mathematically identical to the geometrical optics associated with a certain class of mirror trajectory. Investigation of the simpler system clarifies the relation between particles and energy in quantum field theory. A mirror trajectory is presented by which a flux of particles is created, but no energy at all is radiated. We also show that the stimulated emission that occurs when a single particle is incident on the mirror simply corresponds to the classical reflexion of the associated wave, and that the total energy may decrease in this process.