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A friend on Facebook shared this with me this afternoon…
[Raises his hand and puts on his best Sheldon Cooper impression and says, "Pick me! Pick me! I know the answer!", then launches into an overly long, round-about explanation of the video...]
When I was a young teacher I visited Discovery Place in Charlotte. They had this fascinating laser oscilloscope demonstration. I had to figure out how they made it, so I asked questions. With my seventh grade GT students we built our own laser oscilloscope from an old sub-woofer, a small mirror glued to the speaker, and a helium-neon laser pointed at the mirror and reflected onto a projection screen.
We attached a musical synthesizer to the speaker, and fed various tonalities into the homemade oscilloscope. A sine wave produced a perfect circle. If I played an open fifth on the keyboard, we got an infinity symbol, although it was not quite perfect. A major third produced a pattern that would seem to rotate. The further up the harmonic series we went, the more intricate the pattern we got.
This was a perfect way to demonstrate Bach’s “well-tempererd clavier.” Without getting into too much physics, there is discrepancy between the natural harmonic series and the tonal keys of Western music. If you have a piano perfectly tuned for the key of C, then it will sound out of tune if you try to play something in the key of F. On a well-tempered keyboard, the discrepancy is distributed throughout the keyboard, so the fourths are tuned a bit sharp and the fifths are tuned a bit narrow. The major third bears the brunt of the temperament, and can be out of tune as much as seven beats per second. So, a well-tuned piano is actually slightly out of tune. This makes it so that you can play a piano in multiple keys without having to re-tune the piano each time.
On our homemade oscilloscope, this temperament manifested itself as the rotating pattern when a major third was played, and the slightly imperfect major fifth.
…but, back to the video above….
In the video above the water tube is adhered to the speaker in much the same way as the small mirror in my oscilloscope. It moves in a pattern just like the tone fed into the speaker. That creates the circular pattern in the water stream. The rest, however, is a bit of photographic trickery.
The camera is set at 24 frames per second. This matches the 24 Hz frequency of the sine wave in the video. The result is a stroboscopic effect, where the water appears to be frozen. Increasing the frequency to 25 Hz throws the sine wave slightly out of synch with the camera, so the water stream appears to move forward. At 23 Hz, the stream appears to move backward. I’m guessing that the live view would be somewhat boring in comparison. Of course, you could get a strobe to repeat the effect in real life.
So, that’s how the video was created. I’d love to try it myself. Of course, I’d love to build a new laser oscilloscope. But kids today are making them from new materials. Instead of helium-neon, little laser pointers are readily available, and old CD/DVD players can be cannibalized for their laser diodes. Instead of a speaker, the attenuator arm of an old computer hard drive seems to work better.
So, having explained the joke, and thereby taking all of the fun out of it, I think I’ll go find me a small mirror and glue it to an old speaker.