After watching a movie in which levitating water was used as an illusion, I was curious to discover how it works. Unfortunately I cannot find any great explanations of the science behind the effect, only that strobe lights are what cause it. I want to know why strobe lights cause this effect.

Here is a video of the illusion.

It is my assumption that there is a steady stream of water, and the strobe lights are flickering on and off at a certain rate. Why then do I not see the entire stream of water being lit up, rather than individual droplets? What causes the droplets to appear to "move?" And why are there dark spaces between the droplets when there is apparently water there?

  • $\begingroup$ <I want to know why strobe lights cause this effect.>.. i do not know if you have used stroboscopic effect in labs... but its interesting and the frequency of the flickering light intensity can easily produce illusions of static or slow motion of objects-which are actually in high speed rotation/translational motion. $\endgroup$ – drvrm Jun 15 '16 at 4:30

It is an optical illusion that uses stroboscopic light. It is the same principle that allows you to see still pictures into a movie. For instance, when you see the drops floating still, the drop is not the same, the light frequency is synchronized with that of the the falling drops, so that every time the light is on it shows you the image of a different drop at the same position (when it is off the drop keeps falling and another replaces it). The stream in not continuous, it a "steam" of drops.

If the next dot is actually slightly higher than the previous drop when the light gets on then your brain interprets that it as the same drop moving up.

  • $\begingroup$ So in the case of the stream of water, does it only appear continuous because of the high speeds of the water droplets? Such as in the way a spinning object becomes a blur as it picks up speed? $\endgroup$ – Mitch Talmadge Jun 15 '16 at 4:57
  • $\begingroup$ I do not believe the illusion can work with a continuous stream of water, all of them I have seem use a "stream" of drops. Yes, it will appear continuous in such a case $\endgroup$ – Wolphram jonny Jun 15 '16 at 5:05
  • $\begingroup$ I have seen it done using a continuous stream modulated by a subwoofer (they taped the end of the pipe to the subwoofer so that it moved the stream up and down). The key is to create something sufficiently repetitive that the human eye can fill in the blanks. $\endgroup$ – Cort Ammon Dec 13 '16 at 1:06

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