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.