This thread made me think of something I’ve been wondering about for a long time.
This is it: A central hub with radial projections rotating - eg. a wheel with spokes or a ceiling fan. When it rotates really fast, the projections are a blur. When it’s rotating a little bit slower, so that at a constant gaze the projections are still a blur, when you close your eyes, for a split second, right before your eyes close, you see the radial projections as if they are standing still. Has anyone else noticed this? I have stood at the sides of roads and blinked my eyes slow and fast at the wheels of passing cars. A good place to do this is near a stop light where cars are slowing down. Try it. It never fails.
You only close your eyes once, so it seems that you aren’t inducing a strong stroboscopic effect, but a likely candidate is the Wagon-wheel effect. This is related to how it seems that car tires are going backwards in movies.
I’m guessing, but the last image to hit your retina before you close your eyes will persist for a short time afterwards (this is how animation works without any blurring in the indivual images), and won’t be confused by further motion. I’d compare it to the strobe effect, where the rate of the flashing will affect how motion is percieved. At one rate, motion seems to slow down, while at a slower rate motion becomes jerky, and even slower, the images become disconnected.
I think this has got to be it or at least close to it. The real question should be why do the blades seem to be a blur if you leave your eyes open. At any given instant you are getting an image of fixed blades in a given position, yet somehow when they are put together in quick succession they appear a blur. Apparently closing your eyes stops this effect so you can see the final image clearly.
Your eyes blur the image because they aren’t focused on the wheel spokes. It focuses on the brake drum behind the wheel or something in front of the wheel. If you try really hard to force you eyes to focus on the wheel spokes, you can see the individual spokes turning. I do this all the time with the ceiling fan.
Do you also find that the blades seem to be moving backward? Because it seems like, when I do it, it takes more time to process the image, and I wind up with something like the television/strobe effect.
