From: jeroen@nospam.please
Phil Hobbs wrote:
> On 10/27/20 5:00 AM, Jeroen Belleman wrote:
>> Joe Gwinn wrote:
>>> On Mon, 26 Oct 2020 20:07:16 +0100, Jeroen Belleman
>>> wrote:
>>>
>>>> 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.
>>>
>>> Clear CD boxes are usually polystyrene. and yes that is how that
>>> behaves. So, I agree that polystyrene is ruled out.
>>>
>>>
>>>> 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.
>>>
>>> If you dissolve it in acetone, do you get a powder residue?
>>>
>>> In other words, is the rotation due to the plastic, or to a mineral
>>> filler (if any)?
>>>
>>>
>>>> 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.
>>>
>>> Liquid crystal polymers (LCPs) are typically very tough. But I'm not
>>> sure any are transparent. Or that one would make cheap rulers from
>>> LCPs.
>>>
>>> Wonder if it's polycarbonate, which is transparent like polystyrene.
>>> But I don't recall that it rotates polarization.
>
> Polystyrene generally has a lot of stress birefringence, which is what
> gives the coloured fringes when it's placed between crossed polarizers.
>
>>>
>>> Where did this mystery ruler come form?
>
>>
>> It's Chinese, I can't read the brand name. It's a 'comma shaped
>> French curve ruler'. There are lots of brands that look alike.
>
> To get optical activity, the polymer has to be chiral, like biological
> molecules. (A strong sugar solution is optically active as well.)
>
> Cheers
>
> Phil Hobbs
I now believe the rulers are some polyester. A sheet of mylar
I tried shows a similar behaviour, though not as pronounced.
(It's much thinner too.) I guess the ruler happens to have
just the right properties to work as a half-wave plate. I'll
try a piece cut from a PET bottle tonight.
A 4mm thick sheet of polycarbonate shows a uniform green or
magenta hue, depending on polarizer orientation. A piece
of thin cellophane varies between yellow and blue.
Jeroen Belleman
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
