Recently, my wife and I went and saw Avatar in 3D at the local
cinema. They used RealD 3D glasses that look just like
sunglasses. Rather than dropping ours in the recycling bin after
the movie, I took them back to the lab to play with, because
they have some interesting optical properties. Here’s a view
through one of the lenses of the big LCD display in the lab: At
first glance, it would seem absurd to have light behave so
differently when going forwards or backwards through the
glasses. However, there is an interesting explanation for why
this occurs. The way we are used to experiencing light
interacting with an object is typically symmetrical. If I can
see you in a mirror, then you can see me through that same
mirror. However with the RealD glasses, LCD light can pass
through relatively freely in one direction but is blocked in the
other direction. This effect is only seen when looking at LCD
displays, not at other lights, which is a big hint as to what’s
going on. Further investigation shows that the brig