The "Wagon Wheel Illusion" and what it does to our eyes | Hey Ray!
PITTSBURGH (KDKA) - I met a gentleman at a restaurant who wanted to know why wheels that are moving a vehicle forward sometimes look like they are spinning backward.
While this is a weird phenomenon, when you see how it applies to the motion of movies it becomes simple to understand.
Typically, movies are recorded in 24 frames per second. That means there are 24 pictures strung together to make one second of motion.
Your brain fills in all the gaps to perceive that movie motion. If the wheel almost makes one rotation from one frame to the next in that movie, the brain fills in the gaps the best it can, making the wheel seem to spin backward, even though the vehicle is moving forward because the pattern didn't advance the whole way around.
In old Western movies, this perceived backward look to wheels on wagons became known as the "Wagon Wheel Effect."
The scientific term is the stroboscopic effect.
To illustrate this, I created this disk with a weird pattern on it. You can see, that as I increase the speed of its spin, the circles seem to move differently. I recorded this video at 24 frames per second, just like the movies.
At different speeds, the disk's pattern seems to move differently, at times very slow, and they even appear sometimes stationary, even though the disk is spinning. This illusion happens because of the frequency of the camera taking a frame of video and where the segments are at the time that frame of video is recorded.
Think of it like this: If you set a camera to take a frame of video of a clock every 12 hours, the clock would seem to stand still. If you took a frame of a clock every 11 hours, the clock would seem to move backward. The frequency of the image is important here.
I'm not trying to hypnotize anyone with this, but our brains have sort of a frame rate or a refresh rate when taking in information, too.
Under certain lighting conditions, and depending on the distractions around you, this wagon wheel illusion can occur in real life, too. This stroboscopic effect is most notable when motion occurs near a flashing light. Even lights that flash at a frequency your brain can't detect, like an LED.
Just like a strobe light! It all depends on the motion and the frequency of the light blinking.
The University of California, Davis Computer Sciences department makes a good point about the wagon wheel or stroboscopic effect saying, "Such an effect may be seen as just an illusion, but it should really be considered as a warning: if we do not know the actual physics of a phenomenon, sampling such a phenomenon with the wrong frequency may lead to misinterpretation."