Why do fan blades look slow when tv screen directly behind?
Answer:
Well when fan blades move fast enough the human eye cannot keep up with the rate of motion. This is why they appear to move backwards or stationary or slow. Similarly, movie cameras are even slower than the human eye, so in film when a car wheel or wagon wheel appears to move backwards etc its because the camera frame rate is lower than the speed of the wheel. I assume the television light is simply either accenting this effect or changing it in some way.
The TV acts as a strobe light.
it is because the verticial blanking of a normal tv, is around 60 hz. so it is kind of acting like a timing light. but since there is some slippage in the 60 hz field of the motor, you get either and advancing or retarded motion.
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