Hello. Sorry if my question sounds somewhat weird (I'm a mathematician, not a physicist). I am trying to understand something for my work. I would like to know what is your opinion about it. Assume that there are two electrodes inside a vacuum tube, with a difference of potential between them, and that a flux of charged particles (to make thing simple at first, say electrons) is moving from one electrode to the other. Assume furthermore that a monochromatic microwave (say) is emitted continuously from one electrode, and propagates through the tube in the longitudinal direction to the other electrode. Intuitively, the wave communicates some vibration to the particles, so, when the wave reaches the extremity of the tube and exits the tube, its shape is probably different from the original one. 1) to what domain of physics does this question belong ? 2) what is qualitatively the shape of the wave after it has left the tube ? 3) how does the charge and the direction of the movement of the particles affect the wave ? thx.