From: whit3rd@gmail.com   
      
   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?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)