Reverse the Polarity

[Title from an (in)famous Doctor Who quote]

 

Last night I went to see Avengers: Endgame in 3D, and I can never resist playing with the glasses afterward, so I thought I'd talk a bit about how they work. In order to get a 3D image, your eyes need to get slightly different pictures – You can see this by alternately closing one eye and then the other. For a long time, this was done with glasses that had one red lens and one blue. The two images would be printed in the same red and blue, so each eye could only see one. Unfortunately, this only works for black & white (or in effect, black & purple) images.

Modern 3D films instead use two different polarizations. I've mentioned polarization before talking about sunglasses, and that's basically all the 3D glasses are, with one important difference: Normal polarized sunglasses are linearly polarized, while the kind used for 3D are circularly polarized. You can think of polarization as an arrow pointing perpendicular to the direction the light is traveling. For linear polarization, this arrow is fixed, but for circular polarization it rotates. The polarization in the linear case is usually described as horizontal and vertical, while for circular it is right- or left-handed*.

We need two different images to get the 3D effect, so we could do that with one horizontal filter, and one vertical. The problem is, if you tilt your head, the two images will mix. To avoid that, films use circularly polarized light, with the glasses constructed to filter right- or left-handed polarizations. To make that filtering happen, the light is first transformed into linear polarization, then filtered leading to some interesting effects. Next time you see a 3D movie, I highly recommend playing with the lenses a bit:

Both lenses forward, 90° rotation

One lens reversed, 90° rotation

*In case you're wondering what physicists mean by the handedness of quantities: You're probably used to talking about rotation in terms of clockwise and counter-clockwise. There's a problem with this though, if you imagine a see-through clock. Viewed from the back, the hands appear to be moving counter-clockwise. To remove this ambiguity, physicists take their right hand and curl their fingers in the direction an object is moving. Extending the thumb points in the direction of the rotation vector. In the case of a clock, this points into the wall.