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I recently bought a few pieces of linearly polarized film (the one intended for smartphone LCD screens). At first I was confused because I could not make the polarizers work as they should - no matter, how I combined and rotated them, they did not block any light. All I got was a rainbow-kid effects.

I knew that the polarizers have protective plastic films on both sides of them, but it did not occur to me that these films might somehow interfere. After all, it's just a usual plastic film, nothing more.

To my surprise, after I removed the plastic films from the polarizers, they started to work! Then I tried various experiments with multiple kinds of transparent plastic items I had in my house and was dumbfounded. Almost every piece of plastic somehow interfered with the polarizers when the plastic in question was put in between. Some pieces of plastic showed only rainbow effects (I later found out that these rainbow effects indicate the stress of the plastic). I guess, that's completely normal and expected from everyday plastic when put between two polarizers.

But then some transparent plastic bags seemed to interfere with the polarization, so I had to rotate the plastic item about 45 degrees to get the polarizers work again. On the other hand, when I put a piece of cast acrylic sheet between the polarizers, it did not have any effect at all, like there's nothing between the polarizers - no even the stress rainbow effect I saw for other plastics.

How does it happen that there are so many transparent plastic items (including the plastic film used to protect the polarizers themselves) that happen to affect polarizers and either cancel their effect completely or have to be rotated 45 degrees to prevent the interference?

Does this mean that many everyday plastic materials are somehow polarized? Is this intentional or a side effect of some special plastic manufacturing process or materials? And how is acrylic special, that it behaves just like thin air between polarizers?

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  • $\begingroup$ "Even in a naturally isotropic medium, birefringence can be induced e.g. by inhomogeneous mechanical stress. This can be observed e.g. by placing a piece of acrylic between two crossed polarizers: when stress is applied to the acrylic, one observes colored patterns resulting from the wavelength-dependent effect of stress-induced birefringence." rp-photonics.com/birefringence.html It may be the sheet was too thick to show this? NB also impact of extrusion on optical properties $\endgroup$ – CriglCragl Jun 26 '18 at 12:47
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Many transparent plastics are birefringent, meaning they have two indices of refraction, one for a particular direction of polarization (the direction in which the polymer molecules are aligned) and the other for the orthogonal direction. If a light wave that is linearly polarized in some other direction than these two enters the material, it will be separated into two polarized waves moving at different speeds, which, upon emergence, may no longer be in phase, resulting in a wave with a different polarization than the incident wave.

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  • $\begingroup$ By the way, you should have seen the same effect with the acrylic plastic. Did you try all possible orientations of the acrylic relative to the polarizers? $\endgroup$ – Jon Jun 26 '18 at 1:22
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Birefringence in plastics can be caused in several ways. One is by stretching: stretch a piece of polyethylene sheet, put it between crossed polarizers and hold the sandwich up to a light, and you will see colors that relate to the amount of stretch. Another cause of birefringence is flow. In a clear CD case you will often see colors that result from birefringence, and the pattern of color relates to the flow pattern that occurred when the case was injection molded. Yet another cause is strain, which is closely related to stretching. In most such cases, the birefringence is due to the distortion or alignment of polymer molecules. Cellophane tape is stretched during its manufacturing process, and is strongly birefringent. Wad up a piece of cellophane tape, put it between crossed polarizers, and you will see a beautiful color pattern due to the varying thicknesses and angles of the tape.

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