Saturday, July 18, 2009

Strain patterns in plexiglass.

Polynomial: I am still working on multiplayer, and there is not much to show off. I made a science video, though.

Using regular polarizing sunglasses as a filter (I do not have real polarizing camera filter).

How it works: Light waves are not like sound waves, but more like displacement waves in a stretched rope - the light "waves sideways". Usually light is waving kind of randomly, in all directions. When the light is waving in just one direction, that direction is said to be a 'polarization direction' for the wave. LCD displays emit polarized light, which is usually waving in approximately diagonal direction (I do not know if direction is same for all displays or just for LG displays); polarizing sunglasses only pass vertically polarized light. So when you look through polarizing sunglasses on LCD display, at right angle, no light goes through, because display gives out \ diagonal polarized light, and glasses at that angle only pass / polarized light.

Crystals of various sorts, strained plastics, and similar materials act in a very interesting way. The light waving along the stretch direction moves at slightly different speed in such materials, from the light waving orthogonally to the stretch direction.

If plastic's strain direction is at angle to the polarization direction, the polarization of light appears to flip as light goes through the plastic. Polarization of different wavelengths (spectral colours) of light flip at different rate as light goes through such material (it is flipping again and again). In result, for some wavelengths (colours) the polarization is completely flipped(flipped odd number of times) and the light passes through sunglasses, and for some wavelengths, the polarization is not flipped (flipped even number of times) and no light goes through. That's where colours in this video come from. The colours remind of soap bubble film, and indeed, the spectra of those colours is pretty much identical to thin film interference that you see in soap bubbles or a thin layer of oil/gasoline/etc on water.

For more precise details, see Wikipedia:Birefringence.

(By the way, LCD display itself uses flipping of polarization in liquid crystal (which is put under electrical strain rather than mechanical strain, i.e. electric field) to form the image, but I digress)

Todo: sometime I'm going to make better video with narration and pictures how polarization works.

For now, I recommend simply trying it yourself and showing it to some friends!
Just get some polarizing filter (I'd recommend linear, though circular will work. The polarization in material is actually flipping like horizontal, right circular, vertical, left circular, horizontal, right circular, and so on. Circular polarization is like swinging rope in circle pattern - a rotating wave moves through the rope). Stay away from those uv polarizing filters, those are probably totally irrelevant. You can also get polarized sunglasses, those are better for visual observation.

Actually, you can do this experiment even without buying any fancy stuff. Look in the reflection of display in piece of glass, at approximately 50 degrees angle. Tilt the glass to the side while maintaining reflection angle, until reflection becomes 'dark'. (I did not manage to get a good picture of this with my camera, due to lack of third arm, but it works very well). At some angle, only one polarization of light is reflected. That's why polarizing sunglasses help filter out reflections from water or ice - the glasses let other, orthogonal polarization direction pass, but I digress.

In fact you can even do that without LCD display. Light from blue sky (at the 90 degrees angle from sun) is polarized! So you can impress your friends by showing that off with piece of broken glass, and some plastic garbage. The hidden beauty of everyday things. (Practice first, the good angle might be hard to get right at first)

[BTW. As you can see, I'm still experimenting with ads. I'm very interested in efficiency of ads, naturally, as the time will come when I'll be possibly paying for ads for The Polynomial]


  1. Hey, that's awesome!!! great video, and explanation.

  2. Super cool! I've tried all the plastics I had around my table. Plasma FTW!