Here’s an interesting optical illusion video that uses water and sub-woofer to trick your eyes.
The illusion gets better midway as the droplets of water goes upwards, against the gravity.
Brusspup, who uploaded this optical illusion video on YouTube, explains the trick:
Fill a bucket full of water and place it about 5 feet off the ground. Place a speaker (a really good subwoofer) about 1 foot lower than the bucket. Run a plastic tube from the top bucket down in front of the subwoofer. Tape the tube to the front of the speaker. Then aim the end of the tube to an empty bucket on the floor. Get the water flowing from the top bucket. Now just generate a 24 hz sine wave and set your camera to 24 fps and watch the magic happen.
And this is what you need to do to reverse the flow of water…
Now if you play a 23hz sine wave your frame rate will be off just a little compared to the sine wave causing the water to “move backward” or so as it appears. Basically your cameras frame rate is synced up with the rate of the vibrations of the water so it appears to be frozen or still.
You can play a 25hz sine wave and cause the water to move slowly forward.