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(Please see this gif here) The device seems to spin at a very high speed and we can see the wings of the device turning slowly (IDK due to some illusion). We can encounter this phenomenon in spinning fans too.

My question is, why am I able to see the wings of the device on those particular points? Why am I not seeing them in other points? Does the device tend to slow down at those certain points?

I'm not a Physics grad or anything. Sorry if the question is trivial.

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There a lots of things that go "weird" when you record something with a digital camera. Specifically to your question, you're recognizing the stroboscopic effect (Wikipedia), which happens when you "sample" light with a camera at a specific frame rate. You can see "frozen" motion and even "backwards" motion from different frame rates as shown below:

stroboscopic effect

Another effect in that clip are the wings changing in size because of rolling shutter. Aliasing is another interesting effect, but not seen in the gif. Have fun exploring!

EDIT:

You can even see this inside too, if you have an illumination source that strobes (most things LED, fluorescents sometimes). It will look like this:

hand waving and strobing

EDIT2:

For continuous light, it appears there are some effects that can be seen with the naked eye. See https://en.wikipedia.org/wiki/Wagon-wheel_effect#Truly_continuous_illumination

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  • $\begingroup$ I agree with your answer that is respective to the device (camera). I can encounter this phenomenon even when I am looking directing at the spinning object. I want to know why is that. $\endgroup$
    – Bej
    May 2 '17 at 4:35
  • $\begingroup$ Updated answer with an explanatory image of strobing inside. Unless you are talking about outside... $\endgroup$ May 2 '17 at 17:06
  • $\begingroup$ @Bej The only way you can see the wagon wheel effect with the naked eye, in direct sunlight, is because we perceive reality in distinct quanta. Basically our visual system has a frame rate. If vision was continuous, anything spinning fast would just be a blur. I tested it with a spinner and definitely saw the wagon wheel effect. $\endgroup$ Jun 1 '17 at 19:23
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Vision may seem continuous but it's not. The eye captures a couple of frames per second. If the blade rotates at a rate where every frame has the blade in the same position it looks like the blade isn't moving.

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    $\begingroup$ Do you have any source for this? I've never seen anything that actually says that vision is taken periodically in any capacity that is similar to frames per second. I've seen articles discussing how it's increasingly difficult to perceive higher framerates; but nothing that suggest human vision has an upper limit of optical framerate, just that after a certain frequency there are diminishing returns on the ability to discern the framerate. The other answer seems to really address it, which explains that the device used to capture the .gif had a framerate; not the human's perception. $\endgroup$
    – JMac
    Apr 28 '17 at 14:41
  • $\begingroup$ @JMac You're right: there is no defined frame rate for vision: in particular if you look at things like spinning spoked wheels under daylight (so not under any light source which does have a frame rate) you don't get aliasing except in weird cases where you are physically tracking the object with your eyes: you just stop being able to see the thing clearly. $\endgroup$
    – user107153
    Apr 28 '17 at 15:59
  • $\begingroup$ 10 to 12 images per second en.m.wikipedia.org/wiki/Frame_rate $\endgroup$ Apr 29 '17 at 0:08
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    $\begingroup$ @zanescheepers I still don't see any actual source for this. Perception of motion is extremely complicated. Trying to treat it like digital video may not be justified. $\endgroup$
    – JMac
    May 1 '17 at 13:35
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    $\begingroup$ @zanescheepers That article (and the one it links to) don't seem to give any sources for the claims that the eye actually perceives motion in frames; just that it will perceive a flickering light as continuous. It seems anything about "frame rate" is paraphrased off of much more complicated processes that don't work like digital image processing we use now. $\endgroup$
    – JMac
    May 3 '17 at 22:27

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