Visual Trickery: Loopy Water, Unmoving Hose
This is nuts. In the two-minute video below, YouTube user Brusspup shows us how a carefully tuned speaker, a tube spitting water, and a camera can create an impressive optical illusion -- the illusion that water flows out of a seemingly unmoving tube in 3D spiral shapes.
So how does this work? It's actually pretty simple to set up: the speaker plays a 24 Hz tone (a very low note), which means 24 cycles of the speaker cone per second. A hose carrying water is attached to the front of the speaker, so it moves in time with the tone (thus, it is waving back and forth 24 times a second). This movement causes the hose to spray water back and forth. Finally, a camera running at 24 fps (frames per second) is pointed at the hose's output. The result is what you see below -- when we synchronize the camera's snapshots of reality and the speaker's movement, we see what looks like an unmoving hose -- the camera happens to snap a frame at moments when the tube is in the same place, so its wild movement is hidden. (Then Brusspup shows us the effects of tuning the speaker at slightly different frequencies -- trippy stuff, and you can now see the tube moving.)
What we're really seeing is a series of snapshots of water that happens to look really neat; the naked eye doesn't see this effect because the eye isn't a video camera (it is not limited to a "24 frames per second" view of the world). This effect is related to the reason that car wheels sometimes "go in reverse" when seen on TV or film: because the frame rate of any camera is limited, the motion of an object being filmed can interact with the camera in interesting ways.
With all that mumbo-jumbo out of the way, just look at this:
If you like that, you'll probably dig Strobe Lights & Water Drops, including a "Time Foundation" -- a technique using a strobe light to make water drops appear to "freeze" in mid-air.