A: This is an illusion caused by the way our eyes and brains sample motion in discrete snapshots and the fact that fans have identical blades.
Although a spinning fan changes continuously, we don’t perceive every intermediate position perfectly. Our visual system ‘integrates’ information over short time windows and updates its best guess of where the blades are many times per second. If between two updates the fan has rotated almost, but not quite, 360º, the pattern we see now looks most similar to the pattern from a moment ago after a small shift in the opposite direction.
That is, your brain tends to match the current image to the previous one using the smallest apparent shift, because that’s usually the most plausible, and that shift can be negative, or backwards, rather than positive.
Further, 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 advances by slightly less than an integer number of blade spacings between ‘samples’, it will appear to drift backwards. This can be another cause of the same effect.
