Over the years, a lot of people have been looking at crystalline particles under a microscope (like the DIY Community Microscope Kit), and the idea has come up a few times to try using polarizing filters -- for example to try to distinguish crystalline particles from other types of particles.

See #10493, #17785, and others under #polarized-light-microscopy:

How does it work?

The idea (I think) is that crystals "twist" the polarity of light, and so shining polarized light through them, and then re-filtering it with a second polarizer would... block or allow light differently through crystalline particles when compared to non-crystalline particles.

I'd love to hear more specifics from folks on what's happening here, but in any case the effect is that crystalline particles look really different, and change color pretty dramatically when the polarizers are rotated.

You can read a lot more on this, and see some early tests, here: #10493

Samples

Crystalline #silica is dangerous to breathe in; please be careful, and plain sand may be a safer thing to test this on unless you're specifically looking for crystalline silica. You can use a respirator as recommended on containers of grout, joint compound, cement or mortar.

What I did was to look at this while wet, so there's very little chance of it getting airborne. I got some from washing out some grout from a tile project, but you can collect dust samples too -- #dust-sampling.

Setup

I took these polarizers from a pair of 3d glasses from a movie theater (they appear kind of grey, not red and blue) - I think we used the left and right lenses. I believe in these the left and right are circularly polarized clockwise and counterclockwise, but I don't know exactly what that means; I also have some linear polarizing film but haven't tried it yet.

To get the filters out, I just broke the glasses and carefully pulled out the lenses, which are flexible film.

You can buy these as an add-on in the Public Lab store here: https://store.publiclab.org/products/microscope-addonns

I've drawn the polarizers at 90° angles, but we tried rotating them back and forth to see the color-changing effects pictured. Try different arrangements and report back what you see!

Examples

@peasepoint took some great ones at the last #community-microscope workshop at AS220 in Providence, shown below. Post your own in the comments!