Why does the blue wave when it reaches the edge of the open tube not follow its path instead it reverses red?

enter image description here

  • $\begingroup$ I do not quite understand the connection between the question and the figure. It looks like a frame from an illustration for standing waves like this animation: upload.wikimedia.org/wikipedia/commons/5/5d/Waventerference.gif $\endgroup$
    – user137289
    Sep 15, 2019 at 18:44
  • $\begingroup$ that is what I mean because the green wave is reversed and changes blue, the green wave moves in the right direction and the blue wave in the left direction, where that blue wave comes from, why the blue wave does not follow its path to the right and why is the blue inverted? $\endgroup$
    – molo32
    Sep 15, 2019 at 18:53

1 Answer 1


Because of the boundary condition implicit in the open end. The pressure at the end must be the same as the pressure outside the tube (which is represented here by the straight gray line), so the red and blue must cancel out at the ends.


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