I suppose the following is a standard demonstration. An open ended cylindrical tube is partially filled with water. When a certain frequency is applied, using a tuning fork or otherwise, a clear resonance is heard. The end correction could be invoked to get a better result in determining the speed of sound. Nothing special here (although an analysis of the end correction takes quite a lot of physics, well beyond HS).
However, a very good student recently asked me: how come there is a node at the air-water interface? A sound wave is reflected from the water surface, and, the incoming and reflected waves interfere with each other to produce a node. The trouble is, she noted, that sound propagates faster in water than in air. This means that there should be no phase change in reflection. Hence, the incoming and reflected wave should not be out of phase by half lambda to create a destructive interference (node).
How would you answer the question? Is there a mistake in her reasoning?
Regards,
Antti Savinainen
Finland
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