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This question already has an answer here:

Why surface of water behaves as a denser medium inspite of the fact speed of sound is greater in water than in air as seen in case of Resonance tube.

To achieve resonance again we lower the level of water so that node again forms at the surface of water.

As far my knowledge concerns reflection from denser medium brings a phase difference of π but not in case of rarer medium.

It resembles with the one end closed organ pipe.resonance tube

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marked as duplicate by niels nielsen, Kyle Kanos, Jon Custer, Buzz, rob Dec 9 '18 at 0:35

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Please be more explicit in your question. $\endgroup$ – David White Dec 7 '18 at 6:09
  • $\begingroup$ @DavidWhite I have tried my best presenting the question. $\endgroup$ – Amir RAZA Dec 7 '18 at 9:51
  • $\begingroup$ Your question is answered here but instead of having a wall you now have water $(1000\,\rm kg\,m^{-3}$ and $1500\, \rm m\,s^{-1})$. $\endgroup$ – Farcher Dec 7 '18 at 16:57
  • $\begingroup$ Are you absolutely certain that the phase reversal occurs at the water surface and not at the opening to the outside air? There are acoustic impendence changes at both ends, but only one of them causes a phase reversal. It's not necessarily the speed in the new medium which determines phasing because this is NOT an EM wave; it's a mechanical wave, and they behave differently. $\endgroup$ – Bill N Dec 7 '18 at 17:52
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Whether the reflected wave has a phase difference of π or of zero depends on if you are considering the displacement of the air molecules (as shown in the diagram) or variation air pressure. A diagram of the pressure wave for the same situation would show a node at the open end on top (where the wave is constrained to be at atmospheric pressure) and an antinode at the bottom.

The pressure wave is what you actually hear. I didn't appreciate this until I had a chance to crawl through a giant tube with a standing wave in it at a science centre.

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When a wave travels from a less dense medium to a denser medium, a small portion of the wave's energy is transmitted into the denser medium, but most of the wave is reflected. The reflected wave does indeed go through a 180 deg phase shift for this situation.

For your drawing, the water's surface acts as a closed end on the air tube, as most of the sound striking the water's surface reflects off of that surface. In order to have resonance, there must be a node at the water's surface (closed end of the pipe) and an anti-node at the open end of the pipe. Because there is a node at the water's surface, there is very little oscillation of air molecules at that point. Due to this, it may not be relevant to talk about a phase shift for the reflected wave at the water's surface, particularly since a phase shift is important for wave interference situations, and this situation involves resonance, not interference.

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