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Why do wheels look like they're spinning backwards?



Something that I've been noticing since I was a little kid is that on car commercials, when they are showing the car driving, it often times looks like the wheels are spinning backwards and the car is going forward. It wasn't until recently that this concept was explained to me. What really causes this is the fact that the frames per second of the video camera is slightly greater than the rotations per second of the wheel. This makes it so that in each new frame that is recorded, the spokes on the wheels are a few degrees short of the position that they were in for the prior frame. This causes the illusion that the wheels are spinning in reverse while the car is going forward. This is known as the wagon-wheel effect. 


Also less commonly but maybe a little bit cooler, the wheel can look like it isn't moving at all. This is caused by the same concept, except the spokes of the wheels are in the same position for each new frame, meaning the frames per second of the camera is equal to the rotations per second of the wheel, or that they have a perfect rotational symmetry. This effect is evident in the clip of the helicopter, as the rotor blade spins in a similar way to a wheel on a car. (for more on helicopters, look at my previous blog post) 


If you haven't noticed this before, look out for it the next time you're watching a commercial or a movie. There's a good chance that you could notice the wagon-wheel effect, in effect.


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