You've seen this optical illusion in movies -- but it happens when you're watching in person too. What's really going on?
Above: The reverse-rotation effect, as filmed by 1 Stop Auto Shop It's a familiar scene to most anyone with a television: The wheels of a forward-moving vehicle will appear at first to spin in one direction. The car puts on some speed and, as you would expect, its wheels rotate faster. But then, the something goes screwy. At a certain point, the spin of the wheels appears to slow, slow, slow. Then, ever so briefly, it stops. When it resumes, the spin is in the opposite direction. By appearance, the car should be moving backward -- and yet, forward it rolls.
In Film And Television
This phenomenon is known as the "wagon-wheel" effect. If, like most people, you're accustomed to seeing the wagon-wheel effect in movies or TV, its explanation is fairly straightforward: Cameras record footage not continuously, but by capturing a series of images in quick succession, at a specified "frame rate." With many movie cameras, that rate is 24 frames per second. When the frequency of a wheel's spin matches the frame rate of the camera recording it (say, 24 revolutions per second), each of the wheel's spokes completes a full revolution every 1/24 seconds, such that it ends up in the same position every time the camera captures a frame. The result is footage in which the wheel in question appears motionless:
So when a wheel seems to spin in a direction opposite its actual rotation, it's because each spoke has come up a few degrees shy of the position it occupied when it was last imaged by the camera. This is sometimes referred to as the reverse-rotation effect. If the spoke over-shoots, the wheel will appear to rotate in the right direction, but very, very slowly.
This, of course, is a simplified explanation. The appearance and strength of the effect also depend on things like the camera's exposure time and the design of the wheel, itself. Take, for example, a wheel with 24-fold rotational symmetry. Assuming every one of its spokes looks identical, such a wheel will appear motionless to a camera shooting at 24 frames per second whether it's rotating at 24 revolutions per second, 48 revolutions per second, or even one revolution per second. It doesn't even have to be a wheel. Consider the footage shown here, in which a camera's frame rate and shutter speed match perfectly with the rotational frequency of a helicopter's blades. For the illusion to take shape, all that is required is that a repeating motion be visible intermittently. A similar phenomenon can therefore be achieved with a strobe light, giving rise to the related "stroboscopic effect":
In Real Life
The wagon wheel effect, as seen on film and television, is easily explained. Less clear, however, is why people experience the the wagon-wheel effect not through a screen or by virtue of strobe lighting, but out in the real world, under constant lighting conditions. There are presently two competing hypotheses that account for this effect.
The first, proposed by neuroscientist Dave Purves and colleagues in a 1996 issue of Proceedings of the National Academy of Sciences, posits that humans perceive motion in a manner similar to a movie camera, i.e. not as one, continuous motion, but "by processing a series of visual episodes... the sequential presentation of discrete scenes."
But in 2004, researchers led by neuroscientist David Eagleman demonstrated that test subjects shown two identical wheels spinning adjacent to one another often perceived their rotation as switching direction independently of one another. This observation is inconsistent with Purves' team's discrete-frame-processing model of human perception, which, reason suggests, would result in both wheels' rotations switching direction simultaneously.
A "better" explanation for motion-reversal, Eagleman and his team conclude, is a form of "perceptual rivalry," the phenomenon by which the brain generates multiple (or flat-out wrong) interpretations of a visually ambiguous scene. Classic examples of perceptual rivalry include the spatially ambiguous Necker cube, the hollow-face illusion, and -- one of my personal favourites -- the brain-bending silhouette illusion, famously illustrated by a spinning dancer that seems to switch directions at the drop of a hat.