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Doppler Effect, Light, and Fan Blades

  1. Jun 14, 2007 #1
    I am playing with a small personal fan and I noticed that when the blades are moving at some frequency that "lines up" with my monitor refresh rate frequency, they appear blurry and stationary, but with altered shapes. Not only that, but certain edges of the blades are colored blue, while the other blades appear red.

    I attached a rough drawing of the phenomena. I know this picture is hard to make sense of, but when this little fan is rotating, it seems to make weird clumps of black that are always bounded by the circle of the blades, but are not necisarrily the same shape as the blades. Also this picture is made as if you were me looking at the phenomena as I started writing this post.

    I have been searching the internet for this effect since this weekend. No one knows about this, and I am really curious how this works.

    Attached Files:

  2. jcsd
  3. Jun 14, 2007 #2


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    Welcome to PF, Catface. The Doppler effect can't have anything to do with this, since the light source is neither approaching nor receding. You're seeing a stroboscopic phenomenon. It will appear different from a normal strobe because the light is coming from a different area of the screen and different coloured pixels with each 'flash'.
  4. Jun 14, 2007 #3
    Closely related to "Moiré patterns".
  5. Jun 14, 2007 #4


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    Another point, the distortion of the shape of the fan blades is due to the fact that the screen is not flashing as a whole. Rather the display lines scan down the screen thus you are seeing horizontal cross sections for each scan line of the screen corresponding to the (shadow of the ) fan at slightly different angles as it turns.
  6. Jun 15, 2007 #5
    I understand the scanline thingy.

    I don't understand why the top of the each figure created by the blades is always blurred blue and the bottom of each figure is always blurred red.

    Also, in my picture, the figures are approximately 3/4 opaque, and the edges of the figures taper off like a gradient in photoshop. I can see through to the screen behind is the concept.

    I looked up Moiré pattern (http://en.wikipedia.org/wiki/Moiré_pattern) and see the relatedness, thanks cesiumfrog.

    Also, I didn't realize that the screen flashed not as a whole; this makes the shape of the figures clearer to me, but I still don't feel comfortable with these exact shapes, and the divergent points between the blue and red blur: I think these blurs are related to the "path" of the screen refreshing. thanks jambaugh!

    further note: my picture, as my brother nicely stated, is horrible. It dosen't do this phenomenon justice. I'll try to get this documented, assuming that a camera won't add another demension of weirdness.

    thanks all!!
  7. Jun 15, 2007 #6


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    A still camera won't mess anything up, but a video sure would. It would just be compounding the effect with a new scan phase. By the bye, your first attachment hasn't been approved yet, so we can't see it.
    I didn't realize that the top was always blue and the bottom always red. The blades must in some way be acting as a prism to direct the incoming light according to frequency. I don't know anything about optics, though, so I'll leave that for someone else.
  8. Jun 15, 2007 #7


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    Here's another point relating to this phenomena. Have you seen that TV "documentary" about UFO's and other mysterious phenomena where they claim some new species of organism is caught on video?

    In the video shots you see flashes of an apparently spiraling blur. They never seem to be able to catch them on film, only on video cameras. Well the obvious "mysterious new species" are simply regular winged insects with their wing flaps being distorted by the same scanning strobe effect. The camera reads scan lines in sequence in just the same way as the video screen displays it. I shake my head at the stupidity of some people. What's more I cringe at the mercenary media who give such people attention for the sake of ratings instead of holding to some level of skeptical integrity.

    James Baugh
  9. Jun 15, 2007 #8
    Yea I have seen that show on history or discovery channel. I always thought thoes thigns were unexplained.

    I just looked up this video () and I agree with you: deffinently insect.

    Here is a link to my picture:http://aycu26.webshots.com/image/16225/2000221396030596128_rs.jpg .

    Also I tried it with a fan with ridgid blades and the same thing happened. Two different materials (flat plastic for the first fan; pressed aluminum for the second fan i test) and the same effect. I'm starting to believe that this has to do more with the monitor than the fan.

    I'll work on my evidence too so it's easier to speculate on this.
    Last edited by a moderator: Sep 25, 2014
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