Tourmaline: traditional uses in lab?

[mishima]

Hi, I was reading Dirac (Principles of QM, section 2 page 7) and he mentions using the mineral tourmaline in a polarization experiment. I was looking for some more information on how to prepare tourmaline for use in the lab (or how it used to be used). In general, any home/school experiments possible with tourmaline samples would be appreciated.

 

[jedishrfu]

This page has a discussion on polarization and uses tourmaline for the experiment

http://www.tutorvista.com/content/physics/physics-iv/optics/polarization.php

CAVEAT: You'll probably get a POPUP from an online tutor wanting to help you, but I think its a tutor for hire site.

 

[DrDu]

The physics is the same as that of modern polariser foils. Both are optically uniaxial materials and absorption arises only for polarization being either parallel or perpendicular to the optical axis (which of the alternatives applies depends on the material). This is called dichroism.

 

[AlephZero]

Dirac probably wrote the first edition of the book before polaroid film had been invented (or at least before it was easily available).

Unless you really want to reproduce a 19th century experiment as a piece of history, I don't see there is any advantage in cutting and polishing slices of rock, compared with an easy-to-use sheet of plastic film (or even a cheap pair of polaroid sunglasses).

 

[mishima]

I teach high school chemistry students about the manufacturing process and polymer chem involved in making polarized film, and I just thought it would be interesting to have something else (another chemical structure) to compare them to. I am fairly sure I don't have the tools for cutting nice slices of minerals, however.

 

[AlephZero]

I guess you can buy them ready prepared from a lab supplier somewhere: http://berkeleyphysicsdemos.net/node/607

You might try searching for calcite (Iceland spar) crystals as an alternative demo. A geological specimen supplier might have a large (50 to 100mm) sized crystal that will show birefringence without any preparation.

 

[mishima]

I do indeed have a nice Iceland spar specimen for that purpose (birefringence), and a decent collection of fluorescent minerals for other reasons. :) There is a certain wonder in the fact that out there in the dirt are stones with such interesting properties.

 

[DrDu]

I did some reading and thought about it a little bit more and want to give you a more in detail explanation of what happens:
As you certainly know, tourmaline crystals have a threefold rotation symmetry axis. Tourmaline contains iron ions which arrange in sheets perpendicular to this axis. Some of these iron ions are Fe2+, some Fe3+.

Ideally you would have to cut a planar sheet containing this axis. Now light passing perpendicularly through this sheet and polarized perpendicular to the optical axis will have the field vector in the plane of the iron ions. The field may induce charge transfer from Fe2+ to Fe3+ which leads to strong absorption. On the other hand light polarized along the optical axis won't be absorbed.

Crystal optics is quite fascinating and still not all effects are well understood. E.g. for biaxial and absorbing crystals.