Blog post: swansont: It's Not All Black and White

Incredible art piece dances with polarized light

There's a video in the link with the art, which is a weird combination of kinetic sculpture and optical effects from polarizers. You don't discern the actual motion of the polarizers, but you see a motional effect from the overlap. Sort of an opto-kinetic sculpture.

One thing:

To quote a phrase: that's not how this works! That's not how any of this works! Simple filters don't add linearly (they multiply — two simple 50% filters would block 75% of the light), and polarizers don't block half each. What's actually going on is that one polarizer sets the polarization of the light (filtering the fraction that is cross-polarized), and then the second one blocks more light, both acting according to the Law of Malus

$latex I=I_0cos^2theta$

All the light is blocked for perpendicular orientations of the polarizers, and at varying levels of light at other angles. Randomly polarized light isn't blocked by 50% — the transmitted intensity is roughly 75%. You can see single polarizers in the video, and tell they aren't blocked by half.

The second piece is in color but only has a still shot, so I don't know if this is color filters or polarization with birefringent materials. I've posted static shots of birefringent materials before, both using a static linearly polarized source (LCD); having the background and/or foreground polarization and birefringent material move might make for an interesting display.

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