A newbie plays with cross-polarised photomicrography
Ever since I was a tyke, I’ve been fascinated by the stars. Age 7 or so, I did a school project about the Solar System – complete with crayon-coloured planets, carefully cut to scale from cornflakes boxes. For my efforts I was presented with a copy of Patrick Moore’s “Observers’ Guide to Astronomy”, which I still have to this day. It was the beginning of a life-long relationship (with the stars – not with Patrick, obviously).
Much more recently – just over a year ago – I began collecting meteorites. I’ve seen Mars, Vesta, and the Moon through telescopes – the idea of actually holding a piece of them in my hand still ranks as one of the coolest things I can imagine. I’ve done a bit of astrophotography, and am a keen amateur photographer. Put all these interests together, and you can imagine the ‘Wow!’ I felt when I first saw came across Tom Phillip’s stunning images of meteoritic thin sections. I gotta get me some of those, I thought.
| I ordered a section of Dhofar 922, dug out my little toy Bresser microscope,
and waited for the post to arrive. I was, to say the least,
disappointed – not an awful lot to see, dull browns and greys, none of
the detail or beautiful colours I’d seen. A little research explained
the problem – the images were taken using ‘cross-polarised light’. I
didn’t understand what that was, but it sounded expensive, and would
likely need some serious optical hardware. I resigned myself to
admiring them from afar. A few months later I downloaded Tom’s thin-section screensaver, which prompted me to do a bit more reading about XPL (cross-polarised light). It turns out that the principle itself isn’t really all that difficult. The specimen is placed in the optical path of a microscope, as normal; the clever bit is that a layer of polarising material is placed above and below the slide. |  |
A polarising filter blocks all the light passing through it, except that which is aligned in one particular direction. Imagine running a string through a gap in a picket fence, the far end tied to a tree, the other in your hand. If you wiggle the string side to side, or top-left to bottom right, the waves in the cord will get as far as the fence struts and stop. If you move the string up and down, on the other hand, the waves will happily pass through to the other side. In effect, the filter does the same with light.
Normally if you put two polarisers at 90 degrees to each other, they will pretty much block all light – the first cuts out all but the ‘up and down’ waves, and the second cuts out even those. (You can try it with a couple of pairs of sunglasses – just rotate one lens in front of the other). But with a thin section in between the filters, the minerals in the specimen change the polarisation of the light so that most does, indeed, pass through. The composition and orientation of the minerals determine the nature of the transmitted light, which means components of the rock with different makeup are clearly identified at the eyepiece. (And that’s about the limit of my technical expertise!)
Hmm. That doesn’t sound too hard, I thought – if I could get some cheap materials I could have a go at this...
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