why a fast running fan appears to revolve in the reverse direction of which its really running?

Answer 1

The fan appears to be rotating in reverse direction only because it is observed in the rapidly flickering light. Eg. A ceiling fan in a cenema thetre, appears to rotate in the reverse direction when the light from the cenema screen falls on it. In the normal light the fan appears to be rotaing in the correct direction. This is because of the NON-COORDINATION of the speed of fan and flickering of light. Another example, a wheel rotating in a cenema picture, appears to be rotating in reverse direction. Because, the number of film fram exposed or shown on the screen is 32 per second.

Answer 2

Because the light source in the room is turning on and off at a frequency of 60Hz, especially if it is a fluorscent light.

So this effect acts like a stroboscope (rapidly flashing light) that instantly freezes motion. You can actually have the strobe alias the frequency of a fan and it will appear to turn backwards!

There is a whole field of digital signal processing devoted to this stuff. Look up alising for given frequency. Nyquist found that to not have alising, you must sample at least 2x higher than the highest frequency. That is why CDs must be sampled at greater than 44,000Hz.

You can also see this same effect you describe in cars on the road being filmed or old westerns where the wheel spins backwards. What is happening is that the film is running so slow that the wheel can make a part of another revolution before the picture is taken. This makes the wheel appear to move slower or backwards and it has an alised frequency!

Answer 3

It's also like a baby trying to watch TV. It's brain hasn't developed enough to translate the view into pictures yet. I mean when the TV is showing a reel of a movie, ex.

Answer 4

because your eye can't see it as fast as it is spinning. your brain is interpreting the info that your eyes are sending to it. this results in the fan appearing to spin backwards slowly.