When light flashing rate is the same as a fan’s spinning rate, can you see the fan standing still? I remember reading something like that and can’t seem to find it. Is this incorrect? If so, is there something close to this scenario?


closed as off-topic by user191954, stafusa, Jon Custer, Bill N, Aaron Stevens Oct 18 '18 at 12:02

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Yes. Indeed, suppose that the rate of a strobe light is equal to the frequency of the fan (i.e. the fan takes $t$ seconds to turn 360 degrees while the light flashes every $t$ seconds). Then in a dark room you’d see the fan to be fixed in place (in the position where it was at the time of the first flash). The fan returns to the original position after $t$ seconds, so a light flashing every $t$ seconds would see in the same position each time.

There are numerous other fascinating phenomena possible with flashing lights and moving objects, collectively known as the stroboscopic effect.

Two more things: For the fan to at appear stationary, the duration of the flash should be less than our reaction time in order that we do not detect the movement. Human eyes can detect changes a little above 24 times a second so the flash should be shorter than about $1/24$ or $0.04\,\text{s}$. Meanwhile, the light should flash about every $0.1\,\text{s}$, as an electric fan usually makes about $550$ revolutions per minute.

  • $\begingroup$ @AlexShpilkin I don't understand the edits. You just reworded things. $\endgroup$ – Aaron Stevens Oct 12 '18 at 1:30
  • $\begingroup$ @Aaron Well yeah, that’s the point of copyediting—make stuff easier to read but don’t change the substance. I thought this was more or less the point of allowing other people to edit (otherwise correct) posts, no? $\endgroup$ – Alex Shpilkin Oct 12 '18 at 1:38
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    $\begingroup$ @AlexShpilkin Yeah I guess. I just didn't think the answer was hard to read before. $\endgroup$ – Aaron Stevens Oct 12 '18 at 1:40
  • $\begingroup$ My answer was a bit wordy as I tried to explain in a clearer way without using many scientific/technical words like "strobe" light as OP might have been confused. Well, it's not a bad edit so I don't mind the slight cohesion added. $\endgroup$ – Tausif Hossain Oct 12 '18 at 11:57

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