While observing a cute laminar flow from a water faucet, I noticed something strange. If I set the flow rate of water to about 1 ml/s and place a finger to limit the vertical water column to about 50 mm, a wave pattern shows up on the surface of the water column. Changing the position of the finger up and down modifies the wave pattern.
This phenomenon is a powerful scientific demonstration of a visible standing wave, modulated by the effect of gravity. I had learned that an acoustic standing wave forms in a flute at every few tens of millimetres and that an electromagnetic standing wave in a microwave oven at every 120 mm, but this laminar-flow standing wave is simply fascinating.
