Why the Fan Dances

You're looking at shadows cast by the fan blades in the flickering light of your screen.

The screen blinks on and off 60 times a second or more, like a strobe light.

The flickering of the screen produces flashes of light that "freeze" the blades.

Did you know?

If the speed of the fan matches the flicker of your monitor, it can work out that the monitor flashes each time the next fan blade comes into view.

So, what you see is a series of shadows that look the same. Since the view doesn't change, it looks like a single fan blade standing still.

If the speeds are slightly out of synch, you see the blades moving slowly.

Well, okay.

But why does one side of the fan blades look different from the other?