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Matt: Okay, so I’m driving along. It could be Anytown USA. Sun is out. Gotta have the sun out. I’m wearing my shades. I notice that a lot of cars around me have these weird patterns in the back windows. Geometric designs, mostly circles, but other patterns too. Mostly newer cars (late ’70s and up), but some older cars. So I take my shades off. The patterns go away! Put the glasses back on and it’s op art all over again. I thought it might be all the acid I ate in the ’70s, but it’s too consistent, too regular....

What gives? PPG’s little joke? Anticounterfeit measure to combat the floods of cheap Chinese knockoff safety glass? Or perhaps aliens “tagging” those slated for extermination during the upcoming invasion? Or worse yet, because there are so many of them and because they drive so badly, these mark the vehicles of those who have recently infiltrated planet Earth as the advance guard. Maybe, just maybe I’ve picked up a pair of'‘special” sunglasses that allow only me to see these alien markings, since few other people notice them.

This is SO strange. Please help me before it is too late. Have a nice day. — Stan, e-mailville

I’ll bet you’re the kinda guy who opens a new purchase, locates the instruction sheet, then wads it up fast and tosses it. Did you read the label on the glasses, or did you just grab the first pair that made you look like a ski stud or jet jockey? You’ve got yourself a pair of specs with polarized lenses, popular with outdoorspersons for their glare-killing properties. We won’t tax your brain with the vibrational peculiarities of light waves; we’ll just say that when sunlight bounces off glass (or water or other nonmetallic surface), most of it is reoriented in a horizontal direction. Your lenses are vertically polarized, meaning the horizontal rays are blocked and only the vertical rays reach your eyes.

Think of it as a dog with a stick in its mouth running through a picket fence. Maybe the more vertically oriented dog will make it, but only if he drops the stick. This is the glare-elimination property of polarized lenses. Your acid-flashback car windows are probably coated or tinted with some kind of sun blocker, and variations in the coating will affect the amount of reflected light, creating your wavy patterns. Polarized lenses are also used to detect stress patterns in curved nonmetallic objects, and the patterns also show up as wavy circles. The amount of filtering depends on the angle between your eye and the object being viewed, so when you move your head, the pattern will change. Try tilting your head far to one side (reorienting the polarization of the lenses) and watch the effect disappear. For the price of a pair of glasses, it’s a pretty cheap light show.

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