Why Don't 3D Glasses Work Backwards?

It’s a question that has occurred to anyone who has ever sat in a theater fiddling with their theater-provided 3D shades waiting for the promos to finish rolling up: Why don’t these work if I flip them around? Here’s the physics on io9.com behind the persnicketiness of 3D glasses.

Read more at http://io9.com/why-dont-3d-glasses-work-backwards-1549073549