Wagon-Wheel Effect

The wagon-wheel effect, also called the stagecoach-wheel effect, is an optical illusion in which a spoked wheel appears to rotate at a different speed, or even in a different direction, than its true rotation, sometimes appearing to slow down, stop entirely, or spin backward. It is most commonly noticed in film or television footage of stagecoaches and wagons in Western movies, though any recording of a regularly spoked rotating object, such as a helicopter rotor, an aircraft propeller or a car wheel, can show it, because in recorded media the effect results from temporal aliasing between the object's true rotation speed and the frame rate of the camera. The same effect can also appear under a flickering light source, a related version known as a stroboscopic effect, in which the wheel's ordinarily smooth rotation is visible only in brief intermittent glimpses, and a further version of the illusion can even occur under continuous, non-flickering illumination.