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Originally Posted by krwlz
So what causes the illusion of a rotating object, switching direction of rotation after a certain speed? (Think car wheels, aircraft prop... Can slightly simulate it with the pink dots if one increases the speed enough)
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Your eyes see at roughly 30 images per second. Put another way, you see one image every 1/30th of a second. So let's say we have a car wheel with only one spoke for simplicity. The spoke on the car wheel starts pointing straight up to 12 o'clock, and in 1/30th of a second it's pointing at 1 o'clock and in another 30th of a second it's pointing at 2 o'clock. The wheel will look like it's going forward.
Now let's speed the wheel up. It starts at 12 o'clock. In the next 30th of a second it's spun around to 11 o'clock. In the next 30th it's spun to 10 o'clock. You'll see the wheel as spinning backwards when in fact it's just spinning so fast that it looks like it's going backwards. Wheels that look like they're not spinning are spinning just fast enough that every 30th of a second the spoke is in the same spot.
BTW this is why TV works. It paints 29.97 still images (frames) per second. Since that's REALLY close to the speed your eye perceives as motion, you see the TV picture as moving, even though it's really only a bunch of still images.