My father reported this story over the weekend: A friend was traveling by car, pulled out her iPad while in the passenger seat, and switched it on. Black screen. Flicked it off and on again. No display. Kept fiddling, and eventually resigned herself to a busted device. Until she knocked her sunglasses ajar, and the screen leapt to life.

They were polarizing sunglasses, and the iPad’s LCD contains a polarizing filter. Cross the axes of the two, and you get darkness. (That’s how the screen works: each pixel’s red-green-blue dots are lightened and darkened that way, by a fluid whose crystal orientation can be controlled with a tiny voltage.) Neat little demonstration here:

As readers of this blog probably know, that light-filtering material was the first invention of the very young Edwin Land, and the product on which his company was built. “Polaroid,” the word, referred for many years to that filter—it was a loose contraction of “polarizing celluloid”—and in the first decade-plus of the company, it had nothing to do with taking pictures.

I’ve written at some length about the parallels between Edwin Land and Steve Jobs, and it’s a neat, tiny, fitting, meaningless coincidence that Jobs’s final product depends on Land’s first. Every LCD does, not just Apple’s: If you for some reason are wearing sunglasses while watching your flat-screen TV, and tilt your head, it’ll black out too. Why you’d be sitting in your living room in sunglasses, rocking your head back and forth, is your business.