Wagon Wheel Effect (Illusion)
Ever notice how in movies or television shows, sometimes a vehicle will be moving forward yet the wheels will appear to slowly roll backwards (or stay still, or slowly roll forward)?
In the early days of movies, this was noticed with stagecoach wheels in Westerns, and the illusion was dubbed the "wagon wheel effect."
This GeoGebra construction is intended to demonstrate how that illusion works. The left view represents a wagon wheel rolling clockwise at some rate which is controlled by the slider at the top. The right view represents the images captured by a video camera at 24 frames per second. The yellow point near the edge of the wheel serves as a point of reference.
When the rate at which the wheel spins (in rotations per second) is slightly less than the rate of image capture (in frames per second), the wheel will rotate slightly less than a full rotation from one frame to the next. When we imagine the video playback of those captured images, our brains interpret this as slowed-down rotation in the counterclockwise direction.
We may reason similarly when the wheel rotation rate is equal to or slightly greater than the image capture rate.
The wagon wheel effect may remind you of other illusions that occur when two related phenomena are slightly "out of phase" with each other.