spinning fast – Answer: This is an illusion caused by the way our eyes and brains sample motion in different snapshots and the fact that the fan blades are identical. Although the rotating fan is constantly changing, we do not fully understand every intermediate state. Our visual system ‘integrates’ information over short time windows and updates its best estimate of where the blades are several times per second.

If the fan has almost, but not completely, rotated 360º between the two updates, the pattern we see now looks identical to the pattern from a moment ago with a small shift in the opposite direction. That is, your brain matches the current image to the previous image using the smallest apparent change, because this is usually the most plausible, and that change may be negative, or backwards, rather than positive. Additionally, LED and fluorescent lights often flicker, which can act like a strobe, forcing your visual system to sample the fan at regular intervals.

If the fan moves forward by slightly less than an integer number of blade spacings between ‘samples’, it will appear to flow backwards. This could be another reason for the same effect.