Why does my laser pointer create a weird effect when shined at a ceiling fan?

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Discussion Overview

The discussion revolves around the visual effects observed when a laser pointer is directed at a ceiling fan. Participants explore the phenomenon of how the laser's projection interacts with the rapidly moving blades of the fan, particularly focusing on the perception of the laser's position and the resulting visual effects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant describes the observation that the laser pointer seems to remain on the ceiling even when the fan blades pass overhead, leading to confusion about the visual effect.
  • Another participant suggests that the effect can be explained by persistence of vision, where the human eye continues to perceive an image for a brief moment after it has disappeared.
  • A different participant agrees with the explanation of persistence of vision and notes that the high speed of the fan blades causes the images to blend together.
  • One participant proposes a method to test the persistence of vision by recording the scene and playing it back in slow motion.
  • Another participant questions whether the eye can perceive the speed of the moving image, seeking clarification on this point.
  • One participant compares the effect to LED displays that create the illusion of movement through rapid changes in light.

Areas of Agreement / Disagreement

Participants generally agree that the phenomenon is likely due to persistence of vision, although some seek further clarification on specific aspects of visual perception. No consensus exists on the finer details of the visual effect.

Contextual Notes

Some participants express uncertainty about the mechanics of visual perception and the specific conditions under which the observed effects occur. The discussion does not resolve these uncertainties.

jhe1984
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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.)
 
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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 sense 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
 
I think he's right.
 
Yes, it's almost certainly afterimage.
 
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.
 
If I am not wrong, our eye can't catch the speed of the movement of the object image. If I am wrong please tell me?
 
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.



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