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I looked at a light like the one shown here through a fidget spinner (I believe I was observing near where the "blades" meet the inner portion) and noticed that it appeared purple/blue, particularly when viewed for a prolonged period (I looked at the reflection of sunlight from snow and observed this as well.). I believe this is either an optical effect (the spectrum is convolved with the Fourier transform of the "chopping function or pulse" ) or has something to do with the eye/my perception.

So what's going on here?

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migrated from physics.stackexchange.com Apr 1 '18 at 1:41

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  • $\begingroup$ What color is the spinner? $\endgroup$ – Asher Mar 14 '18 at 6:04
  • $\begingroup$ @asher, good point- if the spinner color is the compliment of the visual artifact, then it is an afterimage artifact of the retina. $\endgroup$ – niels nielsen Mar 14 '18 at 6:26
  • $\begingroup$ @Asher I observed this through a grey one as well as a red one. I $\endgroup$ – Julien Mar 14 '18 at 13:18
  • $\begingroup$ I didn't expect this to be as easy to see as you described, but I found it right away. I'm almost certain it's an effect of the time-response of the eye. Would this question fit better on Biology? We'd be happy to migrate it for you. $\endgroup$ – rob Mar 14 '18 at 22:54
  • $\begingroup$ @rob Perhaps I was naive in thinking my perception could be easily explained by physics. I'm ok with migrating the question. $\endgroup$ – Julien Mar 31 '18 at 19:45
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I totally wasn't expecting to immediately duplicate this with a fidget spinner and the fluorescent light in my office. What a neat effect!

My technique: I hold the spinner quite close to my eye, so that it occupies nearly all of my field of view, and look through the spinning part at my ceiling light. If the spinner is very fast or very slow, I don't see any change in the hue of the light. However there's an intermediate rate where the back part of the fluorescent lamp takes on a patchy blue tint with colored edges. The tinted region is irregular and unpredictable in the same way as a pressure phosphene, if you're familiar with that better-documented effect, and has kind of the same "feel" to it.

Trying to photograph the effect just gives the rolling shutter problem, so that's no use. (No color change in the photo, either.)

At the rotational speed where the color change effect is the most pronounced, I estimate the frequency at which the spinner blocks nearly all of the light to be 10-20 Hz. (How? I let the thing hit a fingernail and listened to it tap; it sounds like thirty-second notes, four to a click, at a good presto tempo.) I've read elsewhere that the human eye can process images at 10-15 Hz. So some sort of biological stroboscopic effect, where the eye has just enough time to begin to respond to the reduced light before the light returns, seems plausible to me.

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  • $\begingroup$ This article (wired.com/2017/05/the-phyiscs-of-fidget-spinners) estimates the frequency at about 20 Hz as well. However, the number of blades on the spinner will affect the pulse width (I think you were taking that into account), but at least we have an order of magnitude estimate and there may very well be a connection to the "response time" of the eye so that number could be relevant. $\endgroup$ – Julien Mar 31 '18 at 19:43

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