Cross-Polarization Demonstrator / Teaching Aid

Crystals show birefringence, also known as "double refraction". In very simple terms, crystals (with some exceptions) have two indices of refraction. This leads to one light ray being slowed down and color-shifted compared to another light ray. The end result is that all so-called anisotropic crystals turn the polarization plane of light passing through them by 90°.
This effect is routinely employed to distinguish crystals from amorphous material, or to detect crystal defects. The setup is simply a microscope with a strong transmission light source, where the crystal is placed between two linear (!) polarization filters. One is fixed ("polarizer"), the other can be turned ("analyzer"). If you then “cross” them by turning the analyzer 90° with respect to the polarizer, no light will pass through. However, through birefringence a crystal in a matching orientation will turn the light again by 90° with respect to the polarizer, allowing it to pass through because the oszillation direction of the wave is now parallel to the analyzer. Under the microscope, whenever any crystal is in the right orientation, it will appear to light up on a field of black.

This is a printable teaching aid to demonstrate this effect in the classroom without a microscope. The polarizing film sheets can be freely turned, acting as polarizer and analyzer. There's also a sample stage (glass disc) that can be turned about 100°. This is enough, because a crystal can't be more than 45° out from a direction that works. If you have a crystal with special directions you would like to point out (like my quartz), consider lining it up against the polarization directions and min/max position of the stage before glueing it in place.
Then you just put the contraption on an overhead projector, or let your students hold it against a light source (e.g. the sun). This should also work with other objects that show some polarization effects, such as rolled or extruded plastic sheets etc.

The two-color version indicates the polarization directions of the filters. Make sure you mark the directions and match them up with the lines before you glue them in. If you go with the single color print, you could mark the directions with a sharpie or even not mark them at all (in any case everything does turn black when the filters are exactly crossed). See the cutaway image for assembly. Make sure the glue you use for the crystal does not polarize (most normal glues don't). Do not align the crystal along an isotropic direction, and of course do not use a cubic sample (unless you want to demonstrate the absence of polarization there). I added a twinned and a cubic specimen to show their (non-)behaviour as well. Best use the printable jig for cutting the film sheet.

BOM:

- 6x M2 heat-set inserts
- 3x M2x10 cylinder head screws
- 3x M2x4 grub screws
- 50x2 mm round glass disc or lens (e.g. from Aliexpress) 
- a linear polarized film sheet (roughly 10x10cm, e.g. from Aliexpress)
- crystal(s) (shown in the pics: quartz, fluorspar, selenite)
- glue