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The Straight Dope



A while back you put down an anonymous writer who asked, "How come when you hold a chopstick in your teeth and pluck it, the TV screen shimmies? Nothing else shimmies." You ascribed the effect to heavy metal poisoning. Well, Cece, I think you dismissed the question prematurely, without trying it. This effect does occur and results from a vibration of the eyes (connected to the tooth bone) at a frequency near that of the vertical scan rate on the TV, producing a visible modulation effect of shimmying, speaking vernacularly. The other objects in the visual field may appear slightly fuzzy, but they don't shimmy. Chopsticks are fine, but if you want to see the effect more clearly, vibrate your jaw or head with an electric vibrator using different speeds while viewing TV. Hope this shakes you. Find that letter and apologize. --Jim Salter, Dallas

I can't stand it. Every time I rummage through the circular file looking for a letter exemplifying the depths to which the Teeming Millions have sunk--believe me, you'd feel the same impulse if you had this job--I come up with somebody who's tapped into some lost truth of physics. As a matter of fact, I did try this silly stunt--once. But not being the kind of guy who believes in doing it with the shades drawn, I used a well-lit room, which made the effect a lot less noticeable. Having returned to the (darkened) lab, I find that, sho 'nuff, the screen does shimmy. To be more precise, it looks as though it had turned into a jiggling sheet of Jell-O. Very bizarre. Had we discovered this in the 60s it might have replaced the lava lamp.

A ripple effect of this sort is characteristic of interference between two wave fronts, in this case the chopstick- or spoon- or crunchy-candy-induced vibration in your skull and the flicker of the TV. The precise mechanism of this interference I leave to the grad students to figure out.

I hope you can explain something. I was looking in the mirror the other day without wearing my glasses, which I occasionally use because I'm nearsighted. I noticed that things that were far away, even when reflected in the mirror, were blurry. When I put my glasses on and looked in the mirror again, everything was in focus. I found this strange. I thought everything should have been in focus without my glasses, because the mirror was close to my eyes and so (I thought) were the reflections. I guess that's why people don't use mirrors for vision correction, huh? --Kirsten Munson, Santa Barbara, California

I knew there had to be some explanation. The reflection is out of focus, even though you're close to the mirror, because you're not looking at the mirror. You're looking at the image in the mirror, a different matter entirely.

You can prove this with a simple experiment. Look at a mirror from a distance of 6-12 inches. With your glasses off, focus as best you can on some distant object reflected in the mirror--say, a bathroom towel on the wall behind you. No doubt the image of the towel is pretty fuzzy, and not just because you haven't cleaned the lint screen on the dryer. Now look at something on the surface of the mirror, such as a dust speck. You'll observe that (1) it requires a noticeable effort to adjust your eyes--in other words, you're refocusing--but that when all is said and done (2) the speck, unlike the towel, is in reasonably sharp focus. This clearly demonstrates (to me, anyway) that when you look at a reflection in the mirror, you're not looking at the mirror's surface.

So what are you looking at? For purposes of focusing, at the object itself (in this example, the towel). Without going into the technical details, the image of the towel in the mirror is out of focus for the same reason that the towel is out of focus when you look at it directly. In both cases the light travels more or less the same distance from the object to your eyes; the fact that in one instance it bounces off the mirror en route is irrelevant. Unless you want me to get out my giant model of the exposed human eye--and it is looking a little bloodshot--I say we leave it at that.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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