Why does a fans blades go practically invisible when the fan is on?
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The fan is spinning fast enough that it’s only in any particular part of your vision for a fraction of the time – and in particular, it’s spinning faster than your vision updates. (Your eyes don’t exactly have a ‘frame rate’, but there is a minimum time window for you to distinguish and the speed of a fast-moving fan blade is well below it.)
They’re going too fast for your brain to fully perceive each individual blade moving, it just becomes a blur.
A bullet moves too fast for your eyes to see it, right? Similar concept.
You brain can only process information so fast, and this holds true for the eyes as well. Your eyes have something similar to a “frame rate,” where they can only send a certain amount of information over the optic nerve at one time (it isn’t a constant feed, and you don’t have a set frame rate; it varies for each color, amount of light, and how focused your attention is). Something moves too fast and you can (usually) tell that something was/is in that space, but it’s moving too quickly for your eyes to lock on to it and give your brain a clear image.
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