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
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?