Hi, I'm rather confused as to how this diagram in my book. I know that in order for a mode to propagate, a standing wave must be formed in the waveguide, and this happens above some cutoff frequency that one designs for. What I don't understand is, below cutoff, what happens? I don't understand what is meant by the waves must be 'in phase'. In phase with what? The picture I've attached is meant to show how out of phase waves are created if the angle is not correct; however, I don't at all see what created these out of phase waves. I also don't understand why there isn't a wavefront travelling in the direction of the ray pointing downwards. My guess is that when the waves reflect (and go in the direction of the downward ray), they need to be in-phase with the waves moving in the direction of the upward point ray so that they don't destructively interfere. If anyone could shed some light on this, that would be great. Thank you.