Why wheels spin backwards

It is an optical illusion in which a wheel appears to rotate differently from its correct position. This entails appearing to rotate slower, appearing stationary, or appearing to rotate in the opposite direction. The name comes from people first seeing this effect on wagon wheels in movies and on TV.

It is relatively easy to explain and straightforward to understand. Video cameras record footage by taking a series of pictures in quick succession. This frame rate is highly significant in understanding why a wheel appears to spin backward at high speeds. Suppose a 24 spoke wheel is being recording by a camera with a frame rate of 24 per second.

In the end, the wheel will seem to be motionless. This is because movie cameras capture still images of a scene at a finite rate usually 24 frames per second and the brain fills in the gaps between these images by creating the illusion of continuous motion between the similar frames.

If the wheel rotates most of the way around between one frame and the next, the most obvious direction of motion for the brain to pick up on is backwards, since this direction suggests the minimal difference between the two frames. But the "wagon-wheel" phenomenon isn't just limited to Clint Eastwood flicks. People experience the effect in real life, even in continuous light. This cannot be explained by stroboscopic or filmic factors.

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. 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 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.