Two questions. First, does it bug you when people write you questions on bar napkins, like I am? Second, as we leave a stoplight in our cars and look at the wheels of the car next to us, we notice that when the RPMs reach a certain point we get the optical illusion of reverse rotation. What causes this? –J.R. Newman, Washington, D.C.
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Basically what you’re seeing is a strobe (i.e., stop-action) effect. A movie camera operates at 24 frames per second. If a wheel is turning at some multiple of 24 revolutions per second, the spokes will be in the exact same position every time the shutter opens. Ergo, the wheel will appear to be motionless (though possibly blurred) when the movie is played back. If the wheel now begins to slow down slightly, it doesn’t get a chance to rotate all the way around to its original position before the shutter opens again. Therefore it appears to be rotating backward.
Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.
