When viewed under tube light , why does a table fan appear to rotate backwards and forwards?

This is a simple illusion, resulting partly from the fact that the human eye along with the whole cortical system can process 10-15 images per second, almost digitally, so to speak, and also because as Rahul Nayak correctly points above, a fan viewed in a tubelight has specific moments when it is registered more intensely than other (owing to the flicker). (Edit: As per Simon Cooke's comment below, the eye can process around 120 frames per second, depending on the image contrast. The effect is primarily just due to the flicker) As an example, let us assume, the fan completes 7.5 rotations per second = 2700 degrees of rotation in all, in one second. Let us say, the eye registers 10 images per second owing to the flicker frequency of the tubelight, for illustration purposes (The actual flicker frequency is higher). Now, the 10 images the eye registers better in the next second correspond to the fan's position in degrees starting from the blade's initial position (zero) in degrees as 270, 540, .... , 2700. This is the same as 0, (360-90), (2*360-180),...,. In other words, this is perceived as the sequence 0, -90, -180, -270 and so on, which is rotation in the negative direction. The eye registers the position of the blades at positions that deceives the system to believe that the fan is rotating in the opposite direction. Also see http://en.wikipedia.org/wiki/Fra... for more about how this affects the fluidity and choppiness of videos depending on their frame rates (fps). This is called as the Stroboscopic effect. Updated Dec 4, 2012 • View Upvotes read less