# Can a spinning fan look stationary when illuminated by a flashing light? [closed]

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 StevensOct 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.
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.