[Physics] Why Do My Fan Blades Produce Color When They Spin

fanvisible-light

I've posted a short video here as a visual aid.

Whenever I put my fan on Medium speed during the night time, it produces a rotating Blue and Orange color design, despite both fan blades being a white color. My primary question is, why are these colors being produced?

I've been experimenting a bit and bizarrely enough, this color effect is only occurring in my room (and disappears in the day time) and nowhere else in the house, so the effect is most likely being caused by the light I use at night (why it's different from other lights in my house is beyond me). I could be wrong though, but just an idea. Also, things to note:

  1. This isn't a camera effect, the colors are even more distinct in person than the camera quality leads to believe.
  2. This effect only occurs on Medium speed and disappears on both Low and High speeds.
  3. If I have the fan on Medium speed and switch it to High, the color effect starts rotating faster in the clockwise rotation (with respect to the observer) before disappearing.
  4. And if it's originally on Medium speed and switched to Low, the colors start to slow down and eventually reverse in their rotation until they disappear.
  5. It isn't as apparent on camera, but the Blue and Orange design appears at speeds lower than the standard Low speed (as in, when I turn the fan off and the blades slowly come to rest). The only difference is that instead of 5 Blue and Orange segments, there are 10 of each, then 15 of each, etc.

Best Answer

You don't have a fluorescent light but an LED light that time-multiplexes different colors to achieve white. The fan blades act as a stroboscope and make the switching frequency of these LEDs visible.

Or... I am wrong! So back to the drawing board: I did the experiment. Most fluorescent lights in my house do not show this effect, at all, but one does, albeit very weakly! So how to explain the difference: I would say that the basic explanation is correct, except that we have to replace electronically switched LEDs with the phosphors on the inner wall of the fluorescent tube. If the different color phosphors used in some of these lights differ greatly in their fluorescent time (i.e. how long they keep giving off light after excitation), then some would give off light only for a short time after they are being excited by the UV light that the mercury atoms in the gas filled tube give off, while others will show phosphorescence for longer. If the human eye could resolve these quick color changes, the tube would, indeed, change color periodically.

Seen trough a stroboscope like the ones that is created by the rotating fan blades, the human eye can now become sensitive for these time dependent color variations. I do not know why some tubes show basically identical color (i.e. have phosphors with indistinguishable fluorescence time) while a few use slower phosphors with markedly different emission timing. I am sure that digging trough the chemistry of the materials used in these tubes will give a detailed explanation.

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