When a car is about to stop, the speed of its wheel reduces. Why do we see or feel that the wheel of the car is moving in the opposite direction? This can also be observed of a fan at home. So why is it?
Answer
I think that the reason for that is related to the fixed framerate of our eye which is 24 frames per second. In each next frame you'll see the wheel turned by some angle relative to the previous frame. If this difference is from 0 to pi radians (0 to 180 degrees) you'll see it in one direction. If during each frame, the angle will be from pi to 2pi radians (180 to 360 degrees) then you'll see it rotated in opposite direction in each frame. That's why the wheels of the car go through at least 3 stages of illusion: rotating in clockwise direction, slowing down up to non-rotating state and then rotate in opposite direction, while the car accelerates or stops. But in reality of course the rotation rate of the wheels in increasing or decreasing constantly.
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