Hi As the word suggests, a monochromatic wave has a single frequency. However, usually it also said in various books that it has a constant amplitude and phase as well, and thus it is coherent. I don't quite see why a monochromatic wave *has* to have a constant phase and amplitude? I can easily imagine an infinite wavetrain varying in both amplitude and phase (not necessarily deterministically) and still being monochromatic. Best, Niles.