Polarising effects from plastics

[chinde]

Wasn't sure if this is exactly the right place for this, but it seemed the most relevant.

Homework Statement

Ok, I've recently started some research for my A-Level project, and I decided to look into the polarising effects of plastics. When two polarising filters are positioned one after the other, but positioned so one is aligned perpendicularly to the other, no light passes through the filters. However, when a plastic with photoelastic properties is positioned in between the filters, it is clear to see the stress patterns by the variations of coloured light that emerge through the second filter.

Homework Equations

The Attempt at a Solution

I've carried out some experiments in my college lab and here are some of the results i received in photographs:

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As you can see from the photographs, its quite an interesting phenomena that happens here. For the light polarised by the first filter to come out of the second filter, the CD case used here must be rotating the light through 90 degrees (or any angle above this at 180 degree intervals after 90). But why the different colours, and why not white light?

The most feasible explanation that I can think of, is that the different coloured components of the visible spectrum must be being rotated at different rates, and the colour which is rotated through 90 degrees is the colour which is seen through the second filter. According to my research, the determination of how close the colours are to each other is dependent on the stress on the object... or rather the change in stress.

However, and this is where my problem comes in, if this were true, then the colours displayed should be the same no matter the orientation of the plastic with the filters. But when I picked several points on the case and rotated it through 10 degree increments, the colours changed and cycled through the visible spectrum, returning to the same colours every 90 degrees. Also another phenomena which I can not explain is that of the points that I monitored, even when the plastic was rotated in the same direction, some points cycled through the spectrum on direction (Blue --> Yellow --> Red) and others cycled through the spectrum in the opposite direction (Red --> Yellow --> Blue).

I'd be very grateful if anyone could offer any suggestions as to how the stress patterns actually come about, or how some of the points I have mentioned in my last paragraph actually occur. If alternatively I have posted this in the wrong place, then also please let me know .

Thanks,
Chris

 

[berkeman]

I believe that the keyword for you to research is birefringence:

http://en.wikipedia.org/wiki/Birefringence

http://www.exicor.com/literature/birefringenceTutorial.aspx

http://www.microscopyu.com/tutorials/java/phasecontrast/opticaltrain/index.html

And keep in mind that the object between the two crossed polarizers does not need to rotate the light's plane of polarization by a full 90 degrees. Any rotation at all will allow some light through the system.

 

[chinde]

I think I understand what is in the links you've posted up, thanks. I'm having a little difficulty in trying to relate it to my situation though.

Am I correct in thinking that the CD case is anisotropic and is therefore refracting light along two separate refractive indices? Would the CD Case be refracting different wavelengths across the two refractive indexes to give the various colours, or is it the phase difference between the resultant ordinary and extraordinary rays which are giving this effect?

Thanks,
Chris