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I saw that video when they connected a water hose to a speaker emitting 24 hz sine wave, and its illusion on the shape of the follow going out of the hose (because the video capture rate is fixed at 24 fps)

I wonder if I can obtain similar effects with much lower frequencies i.e. (8-10 hz) and if possible what can I use to simulate the effect or do the experiment.

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The frequency 24 or 25 Hz is linked to the number of pictures of the camcorder per second, see physics.stackexchange.com/q/56802 - So if you want to "see" lower frequencies you need a "slower" camcorder. –  Luboš Motl Mar 28 '13 at 11:09
    
Why was this voted down? –  Rafael Reiter Mar 28 '13 at 13:40
    
@RafaelReiter because the water's flow-form is not affected, only that the camera sees specific points in the flow. The video references is an illusion –  New Alexandria Mar 29 '13 at 23:15
    
What I previously understood is that; the flow-form changes into sine wave like flow, and the camera effect is that when shooting with 24 FPS camera in the video the water freezes or going forward under 25 Hz or going backward under 23Hz. But what you are saying now is that the water flow is unchanged and even the flow-form is a camera illusion I don't really understand how is this possible. –  Amr Abd El Ghany Mar 30 '13 at 20:27

3 Answers 3

You can get the same effect with any frequency you want. Take a bicycle wheel and cover it up with construction paper. Then cut out a hole in the construction taper so that when you spin the wheel and look through the hole, you see a snapshot once per revolution of the wheel.

That's all the camera is doing. It is taking snapshots at a certain frequency. If that frequency is about the same as the frequency at which the spout is wiggling, you will see something like that video.

If the frequency is low enough, you could just cover and uncover your eyes with your hand - same thing.

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thank you very much I will certainly try it. –  Amr Abd El Ghany Apr 4 '13 at 8:12

If you want the same effect without a camera, you could use a strobe light, like the MIT Water Piddler. (video).

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A reminder to all that the referenced video is an illusion, where the video capture rate is 24fps, and thus is in sync with the 24 hz wave.


You can use a rapid-capture app for a mobile device, like FastCamera for iPhone.

This app lets you set the capture rate so that you can experiment with various frequencies.

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what if the frequency I will use will be 10-12 Hz, and the human eye sees 10-12 FPS would I get the same effect to the human eye. –  Amr Abd El Ghany Mar 30 '13 at 20:31
    
citation needed on human eye seeing 10-12fps. I think not, else 45-120 fps gaming monitors would be useless –  New Alexandria Mar 31 '13 at 16:42

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