How to Conquer Gravity (Weekly Optical Illusion Explained)
On Friday, we posted a video by YouTube user Brusspup that showed how to create the illusion of water freezing in mid-air. Is it a “glitch in the Matrix,” as CBS News referred to it as? No, it’s just simple science.
The sound waves from a speaker will cause water droplets to fall in a uniform pattern. When you combine that with a camera that follows the same frame rate, the droplet will appear to freeze in real air. As much as we’d love to say it’s a “glitch in the Matrix,” in reality – say the frame rate is 24 frames per second – while it seems as if the droplet is frozen in mid-air, 24 new droplets have actually fallen from the tube in a nearly identical shape and occupied an identical point in space every 1/24th of a second.
This is a similar illusion achieved by a strobe light. The number of times a strobe flashes per second corresponds with a camera’s frame rate, and the illusion succeeds because the human eye essentially functions like a camera. It processes vision by interpreting information from visible light to build a representation of the surrounding world. To explain that in layman terms, a good example is how movies are projected onto a screen. Frames of pictures are flashed across the screen rapidly in succession, and the human brain arranges these frames into a live-stream video. This is all thanks to two perceptual illusions – phi phenomenon and beta movement – or, more disputably, persistence of vision, the phenomenon where the eye retains a visual image for a fraction of a second after the source has been removed.
However, that isn’t the only effect the video shows. By changing the level of hertz from the speaker, you can also change the direction of the water flow (or at least the appearance of this). For example, if you tune the subwoofer to a lower frequency than the frame rate, the water will appear to flow backwards.
As Brusspop explains how to do the effect yourself:
“This is really simple but had such an awesome effect. Fill a bucket full of water and place it about 5 feet off the ground. Place a 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. 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. Now if you play a 23 hz 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. You can play a 25 hz sine wave and cause the water to move slowly forward.”
So while the effect may all be in your head, everyone loves a good magic show – with a little help from science.