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

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May be refraction

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... read more
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

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 a fan viewed in a tubelight has specific moments when it is registered more intensely than other (owing to the flicker).

Grasp Maths

It is just illusion or perception

Promising Tutor

Electrical lamps emit light according to the frequency (50 Hz) of the main supply (that is, the lamp is on for 10 milliseconds, goes off for the next 10 milliseconds, and the process repeats 50 times a second). But we don't see the on-off processes because of our eye's persistence of vision. (The eye... read more
Electrical lamps emit light according to the frequency (50 Hz) of the main supply (that is, the lamp is on for 10 milliseconds, goes off for the next 10 milliseconds, and the process repeats 50 times a second). But we don't see the on-off processes because of our eye's persistence of vision. (The eye has the ability to retain the impression of an image for short time even after the image has disappeared). But sunlight and candle light are continuous without periodically going off and on. Hence when the light goes off, the eye involuntarily retains the image of the fan blades’ or wheel spokes’ position. Again when the light comes on, the fan’s blades would have moved to a new position and the eye records a new image. Depending on the speed the fan or wheel, the image retained by the eve gives us the illusion. read less

Tutor

Tube Light emit light according to the frequency (50 Hz) of the main supply (that is, the lamp is on for few milliseconds, goes off for the next few milliseconds, and the process repeats 50 times a second). But we don't see the on-off processes because of our eye's persistence of vision. (The eye has... read more
Tube Light emit light according to the frequency (50 Hz) of the main supply (that is, the lamp is on for few milliseconds, goes off for the next few milliseconds, and the process repeats 50 times a second). But we don't see the on-off processes because of our eye's persistence of vision. (The eye has the ability to retain the impression of an image for short time even after the image has disappeared). But sunlight and candle light are continuous without periodically going off and on. Hence when the light goes off, the eye involuntarily retains the image of the fan blades. Again when the light comes on, the fan’s blades would have moved to a new position and the eye records a new image. Depending on the speed the fan, the image retained by the eve gives us the illusion. Supposing we make a fan rotate at a speed of 50 rpms and see it under a tube light, the fan would appear to be stationary. If the speed is different from 50 rpm, the fan seems to rotate slowly forwards or backwards depending on the speed. read less

Only when lit by electrical lamps, table fans seem to rotate forwards and backwards. These are a result of stroboscopic effect. The illusion does not occur when the fan is lit by sunlight or candle light. This naturally leads us to a fundamental difference between these light sources. Electrical... read more
Only when lit by electrical lamps, table fans seem to rotate forwards and backwards. These are a result of stroboscopic effect. The illusion does not occur when the fan is lit by sunlight or candle light. This naturally leads us to a fundamental difference between these light sources. Electrical lamps emit light according to the frequency (50 Hz) of the main supply (that is, the lamp is on for 10 milliseconds, goes off for the next 10 milliseconds, and the process repeats 50 times a second). But we don't see the on-off processes because of our eye's persistence of vision. (The eye has the ability to retain the impression of an image for short time even after the image has disappeared). But sunlight and candle light are continuous without periodically going off and on. Hence when the light goes off, the eye involuntarily retains the image of the fan blades’ position. Again when the light comes on, the fan’s blades would have moved to a new position and the eye records a new image. Depending on the speed the fan, the image retained by the eve gives us the illusion. Supposing we make a fan rotate at a speed of 50 rpms and place it under an electrical lamp, the fan would appear to be stationary. If the speed is different from 50 rpm, the fan seems to rotate slowly forwards or backwards depending on the speed. read less

B.Tech(EC/EE/IN),Math(IX,X,XI,XII std.),Physics(IX,X,XI,XII std.) CBSE/ICSE/IB.

a result of stroboscopic effect.

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 a fan viewed in a tubelight has specific moments when it is registered more intensely than other (owing... read more
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 a fan viewed in a tubelight has specific moments when it is registered more intensely than other (owing to the flicker). read less

Physics Professor

I think reason Fan appears to start rotating reverse is not because of the limitation of sampling frequency of human eye. The effect is never observed in the broad day light but only in tube light. the reason is that tube light flickers at about 50 to 60 Hz depending upon the frequency of the power supply.... read more
I think reason Fan appears to start rotating reverse is not because of the limitation of sampling frequency of human eye. The effect is never observed in the broad day light but only in tube light. the reason is that tube light flickers at about 50 to 60 Hz depending upon the frequency of the power supply. So if the Fan rotates at any frequency more than twice of 50Hz (nyquist shanon sampling theorem) it can not be appropriately sampled (the effective sampling rate here is equal to the rate at which tube light is flickering). Helicopter blades on the other hand are always observed in daylight thats y similar the effect is not observed. read less

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