1. The problem statement, all variables and given/known data Hello, For two waves to be coherent, they must have the same frequency right? Does this on its own implies a constant phase different between any point on one wave and any on the other. So, for example, if we had two waves with different wavelengths and velocities but equal frequencies travelling in opposite directions towards each other, and both had the same amplitude, would a stationary/standing wave be produced? A friend of mine said no, but I thought as the frequencies are the same, there is a constant phase difference between any two points as aforementioned, so a stationary wave would be produced. They didn't really explain why hence I turn to you :) Thanks! 2. Relevant equations Not really any relevant equations for this I know of (or at least, at A Level in the UK, there aren't any equations we learned). 3. The attempt at a solution I thought that, instead of looking at waves, if we look at two points, one on each of the two waves concerned, it shouldn't matter if they have differing wavelengths and velocities, they will still have a constant phase difference if they have equal frequencies, because this means they take the same time to go through one complete cycle of the wave, hence difference between the fractions of the wave cycle passed will always be the same?