From: jeroen@nospam.please   
      
   whit3rd wrote:   
   > On Monday, October 26, 2020 at 8:58:48 AM UTC-7, 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 ...   
   >   
   >>> There are quite a lot of optically-active plastics.   
   >   
   >> 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.   
   >   
   > I think that means it's birefringent, i.e. has an orientation (probably   
   > because the polymer was stretched in one direction as the sheet   
   > was rolled out).   
   >   
   > When linear polarized light has E-field parallel to the orientation,   
   > the film is N wavelengths thick.   When it is perpendicular, the film   
   > is N+1/2 wavelengths thick.   There are four inbetween orientations   
   > that correspond to quarter-wave mismatched in two components, that make the   
   > linear polarized light into circular polarized.  Circular polarized isn't   
   blocked   
   > by the second linear polarizer.   
   >   
   > Inexpensive acetate is the most likely material for a transparent ruler.    
   Two layers   
   > of acetate laminated around a printed film with the markings, perhaps?   
      
      
   Thanks for your comments. I'll do some more experimenting.   
      
   The linearly polarized light is still linearly polarized   
   after passing through the ruler, because there are still   
   two orientations where the second polarizer blocks all   
   light. They're just different orientations.   
      
   Jeroen Belleman   
      
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