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This a phenomenon which I notice everyday but never really took an interest in.

Why does a fan (or any other rotary object), when spinning really fast, 'appear' to slow down, upto a certain rpm, then start rotating backwards? If this is some kind of optical illusion why does it reverse at a certain rotary speed?

Can it be due to the asynchronous(fluctuations) velocity which tend to produce after-images in a place other than the one, one would expect the blade to be in at that particular time? But still the question would be about the rotation sense reversal? Could someone pitch in? Thanks in advance!!

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    $\begingroup$ You can always look up in the net or read Wikipedia before asking here. en.wikipedia.org/wiki/Wagon-wheel_effect $\endgroup$
    – Rajath Radhakrishnan
    Commented May 7, 2016 at 6:14
  • $\begingroup$ I didn't know the exact term for this. But, much appreciated. $\endgroup$
    – Abhinav
    Commented May 7, 2016 at 6:36
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    $\begingroup$ It's called aliasing. $\endgroup$
    – Peter Diehr
    Commented May 7, 2016 at 11:11

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You don't see continuously, you see in a series of frames. If a blade does slightly less than one revolution e.g. 350 degrees between frames it looks like it's gone 10 degrees backwards.

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  • $\begingroup$ Its would have been better if you had explained that we see in series of frames because of flickering of tube light and not because of persistence of vision. $\endgroup$
    – Razor
    Commented Aug 19, 2017 at 9:13
  • $\begingroup$ But why do we see in a series of frames? $\endgroup$
    – Jossie Calderon
    Commented Nov 21, 2018 at 20:01

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