Why do the wheels of a vehicle moving forwards sometimes appear to spin backwards, slow down, even stop completely, and reverse direction? We've all witnessed the Wagon Wheel Effect (alternatively, or stagecoach-wheel effect, stroboscopic effect), whether it be on our screens or in real life. And I know you're dying to find out why this happens.

On Camera

Cameras record footage taking a lot of pictures very quickly, and these pictures are played back in very quick succession. This the standard frame rate is usually 24 frames per second, and at this frame rate, images played in quick succession are not easily distinguishable from real motion.

Imagine a wheel spinning at 24 revolutions per second. The spokes of wheels have a strangely perfect symmetry to them such that as the wheel completes a full revolution every 1/24 seconds, the camera will take a picture in perfect synchronisation with the wheel. As you can guess, the wheel would appear to not spin at all.

Now imagine a wheel that is slightly more or slightly less than the frame rate of the device capturing the motion. As you can imagine, the explanation for wheels sometimes appearing to spin backwards can be explained once again by the spokes, in terms of how quickly the wheel spins compared with the frame rate of the camera. If the wheel has a higher rpm than the frame rate, the wheel will appear to be moving forward very slowly. If the wheel has a lower rpm than the frame rate, the wheel will appear to be moving backwards very slowly.

In Real Life

Okay, that explains why when I watch a video of the vehicle, the wheels appear to spin funny. Why then, do I also observe this phenomenon in real life?

"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." - Dave Purves (neuroscientist)

Some scientists have linked this strange phenomenon to the brain rather than the visual cortex. Another interesting phenomenon you might like to read about is The Name Letter Effect.

More research is needed to come to a more scientific conclusion. Let me know if you have any ideas in regards to why this phenomenon happens!

Sources

Crystallinks
Gizmondo
WIkipedia
Geek
Quora