Why on an old wagon the wheel spokes appear to be going backward?
First of all, if viewed live (not on tv or in a movie), they do not appear to be going backward. A tv (or movie) is taken by a series of pictures (called frames) and the speed of these pictures varies by the camera being used. If you could vary the speed of the frames forward or back you could change the direction the wheel appears to be turning.
Therefore it is and illusion and mostly appears on tv related scenes and is a function of the camera shutter speed and tv scanning speed if they were exactly the same the wheel could even appear to not turn. Slight differences in the sycronization of these two (mechanicanical) gives a slow turnappearance to a wheel that is turning fast.
That is my statement and I'm sticking to it.