like watching a call wheel or bike wheel etc
at times the wheel looks to be spinning forward, then looks like its spinning backwards. Hope I explained it enough lol
In: Physics
It’s called ***aliasing***, or the [wagon wheel effect](https://en.wikipedia.org/wiki/Wagon-wheel_effect) and it only happens under artificial lights or on film. **It’s basically a strobe light effect** that happens when the rotation takes just a bit more or less time than the frame rate of the camera or the pulsing of the light source. Artificial lights flicker, eg normal light bulbs turn on and off 60 times per second in North America because thats the frequency of AC electricity coming from the wall.
Anyway, when you have a light source turning on and off many times per second, or a camera filming many frames per second, aliasing can happen when the wheel turning speed is just a bit faster or slower than the time between frames or light pulses.
So imagine a wheel spinning clockwise, so fast that it completes 95% of a rotation between each frame of video. [Here’s what those frames would look like!](https://www.japanistry.com/wp-content/uploads/2017/09/Aliasing-Wheels-v01.jpg) See how based on those, it looks like the wheel is spinning slowly backwards, when it’s actually spinning quickly forwards.
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