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Someone please explain this

  1. Feb 15, 2006 #1
    Just got a laser pointer w/ 5 modes (arrow, dot, line, etc..).

    Anyway, when I shine it up at my ceiling fan, something weird happens. Obviously, every time a blade passes directly over head the image blinks on the blade, then quickly goes back to only being on the ceiling.

    But here's the weird part. I'm almost certain that - even though the blades are passing over - the pointer always stays on the ceiling behind the blade (apparently, even when it's on the blade as well [I see two things then]).

    You'd think that the cycle would just be simply reversed (the blade lights up and the ceiling's off; the ceiling lights up and the blades off), but as best I can tell, that's not what's happening. If it is happening, it is definitely not as defined as the blade blinking. I've gotten on my bed so I can see the ceiling without looking through the fan and I've come to the same conclusion.

    But this doesn't make sense. What's up?

    (PS - I've also tried switching angles etc. And since the pointer can project larger images like arrows, it's easier for me to test than if it were only dots.)
  2. jcsd
  3. Feb 15, 2006 #2
    I think this can be explained through the Persistance of vision. When we view an object, and if suddenly the object is removed, we will sill continue to sence that the object is still there (till about a sixteenth of a second). Hence if the object is repeatly replaced within 1/16 of a second, they seem to 'blend' into each other.
    Now to your question. Since the fan is rotating at a vey high speed, the cycles (the blade lights up and the ceiling's off; the ceiling lights up and the blades off) occur in less than 1/16 of a second. Hence the images seem to blend into each other. That is the simple explanation.
    If you switch off the fan (and rotate it slowly with your hand), the "Weird part" will not be observed
  4. Feb 15, 2006 #3


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    I think he's right.
  5. Feb 15, 2006 #4


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    Yes, it's almost certainly afterimage.
  6. Feb 15, 2006 #5


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    For a simple test of that, which will undoubtedly prove it correct, just take a brief video of the scene and play it back in slow motion or frame-by-frame. In fact, you can determine your personal persistence of vision duration by varying the playback speed until the dot quits disappearing.
  7. Feb 17, 2006 #6


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    If im not wrong, our eye cant catch the speed of the movement of the object image. If im wrong please tell me?
  8. Feb 17, 2006 #7
    I agree - this is almost certainly persistence of vision - there is no physics reason why this could occur that I can think of. the effect is like the clocks I sometimes see advertised where a single line of leds waves back and forth,. showing the time.

    Joe Andersen

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