From: jeroen@nospam.please   
      
   On 2020-10-26 18:33, Joe Gwinn wrote:   
   > On Mon, 26 Oct 2020 16:58:44 +0100, Jeroen Belleman   
   >  wrote:   
   >   
   >> On 2020-10-26 15:57, Phil Hobbs wrote:   
   >>> On 10/26/20 9:35 AM, Jeroen Belleman wrote:   
   >>>> While playing with polarizing filters, I found a plastic ruler that   
   >>>> turns out to rotate the polarization angle of the light passing   
   >>>> through it by almost exactly 90 degrees. The ruler is of a 1mm   
   >>>> thick colourless plastic, rather flexible, clearly not the usual   
   >>>> polyethylene or polystyrene. It does not itself polarize light, it   
   >>>> just rotates it. The angle of rotation does not seem to depend on   
   >>>> wavelength. It's made in China.   
   >>>>   
   >>>> What plastic could this be?   
   >>>   
   >>> There are quite a lot of optically-active plastics.  Optical activity   
   >>> is normally strongly dispersive--what wavelengths did you use?   
   >>>   
   >>> Cheers   
   >>>   
   >>> Phil Hobbs   
   >>>   
   >>   
   >> Just natural, white light. Some plastics show coloured fringes   
   >> when inserted between two polarizing sheets, which is sort-of   
   >> what I expected. This Chinese ruler is special: It rotates   
   >> the polarization. Inserted between two parallel polarizers, it   
   >> has four orientations spaced by 90 degrees where it blocks the   
   >> light. But it is not itself polarizing: I see no intensity   
   >> variations when looking through it with a single polarizer, nor   
   >> when superimposing multiple layers of the same plastic.   
   >> There are no colours.   
   >   
   > It's very likely to be polystyrene.   
   >   
   > Joe Gwinn   
   >   
      
      
   Polystyrene typically shows coloured fringes, stresses frozen in   
   the material, does it not? The transparent plastic CD box I have   
   here, presumably polystyrene, certainly does. The Chinese ruler   
   is different. There are no visible stress fringes. It just rotates   
   the polarization by basically the same angle for all colours   
   everywhere. It doesn't 'feel' like polystyrene either. Too flexible   
   and rather tough, not brittle.   
      
   I'd like to find a clean sheet of the stuff, so that I can make   
   some more serious measurements. My wife objects to me cutting up   
   her rulers.   
      
   Jeroen Belleman   
      
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