Light has always been a passion from its science to its art. As a child, I was given a small triangular glass prism that had belonged to an aged aunt and was intrigued by how light from a slit in a door could be made to produce a rainbow on the opposite wall. However, for the life of me I could never detect the indigo in the so-called seven colours (red,orange,yellow,green, blue, indigo and violet) and the innumerable intermediate shades.
Later I learned that this was a tiny window in the vast electromagnetic spectrum through which we viewed this world and those beyond…the stars and galaxies. I grew to be fascinated by spectra and from ages 13-15 constructed a series of spectroscopes using pieces of replica diffraction grating mounted in 35mm cardboard frames then available from Proops brothers of Tottenham Court Rd., who had an Aladdin’s cave of stuff. In fact it was an interest in spectra and how they related to atoms that got me into physics and into quantum mechanic in particular and then to research… but I never lost the love of pretty colours!
I mention this as a pre-amble to the fact that my younger brother Peter has been here with us for a month after completed the first year of his Earth sciences degree… after life as civil engineer (open cast mining) and jazz guitarist he has followed a passion for fossils and in his fifties is a student again. I had fossils my father brought home from the mines with him and Peter took up collecting with a vengeance. In recent years he has located ichthysosaur and plesiosaur bones in rock falls on the S. Wales coast. Peter, like me, gets obsessed with his interests and he brought with him a box of slides of rock sections. In fact, we are two very big kids!
For those who do not know it is possible, by means of cutting with diamond saws and then polishing with finer grades of abrasive powders, to create slivers of rock so thin they become almost transparent – in fact typically 30 microns thickness (30 millionths of a metre). The slides are not cheap – a tenner or more but then they are laborious to produce and incredible value in the sense that you get a gallery of images from each one by moving a slide a creating your compositions from portions.
Under a microscope they look to be nothing special until, that is, the magic ingredient of light – in this case polarised light – is brought into the equation. Light is first plane-polarised using a polaroid filter beneath the specimen and then another is held between the specimen and camera or microscope lens. This second (the analyser) is rotated until the polarisers are almost at crossing point (near extinction) where the second polariser has its axis of polarisation at right angles to that from the first and, in theory, cuts out the light.
I have covered polarisation in blog posts such as this one . But this is not a post about the physics of material or the constituents of minerals it is about pretty colours for, as those polarisers are rotated relative to one another they reveal a hidden wonder in the crystalline structure of the rock. Some of these crystals are what is called bi-refringent that is they refract light along two distinct planes. The splitting of the polarised light into its constituent colours produces colours in the rock samples – in fact geometric modern art.
All the illustrations here are made from rock sections obtained from GEOSEC slides beautifully prepared by Rob Gill.
When a light wave propagates through space there are both electric and magnetic fields associated with it and they vary at right angles to the direction of travel. They are, after all part of the great family of electromagnetic waves – we usually mention just the “E” field for the magnetic “B” field is just at 90º to it.
Light is said to be plane-polarised when just one field direction is selected. When light is reflected from a non-metallic surface there is a degree of polarisation – sun specs cut out reflected light for they have a film in the lens that acts like a grid (it has long straight molecules) and lets only light rays in a particular direction. The process that produces the blue of the sky is the scattering of light from gas molecules in the upper atmosphere and this also creates a degree of polarisation. When sunglasses or a polarising filter are rotated you can see how this is cut out and the sky darkens.
In elementary science classes you may have been shown how first polarising light, passing it through a piece of per spec in a clamp or crumpled cellophane reveals colours when you look through another polariser – the process can be used to reveal strains in transparent materials. By rotaing one of the polarisers you change and intensify the colours.
If the sample between the polarisers is a very thin rock section then the crystals in the rock can polarise the light by reflection within their structure. There is a another process at work her called bi-refrigence where a crystal refracts light differently along two distinct axes and the light is split – hence the colours that appear.
An adhoc set up
I could not resist trying to get some images of the slides Peter had brought and dug out two old circular polarisers (same as linear but with a layer that shifts the phase of the polarised light by 1/4 of a wavelength. If you use these they have to be both the same way around…
A table lamp was used for focus and a flash run from the camera via DTTL was the main light source. I made a simple wooden stand and rotated the polariser until the maxium colour was obtained and went from there…
By the way the lenses I used were a pair of Nikon microscope objectives (x4 and x10) that cost me about £50 each and are exceptional used on a bellows. In fact they are as good as the Zeiss luminars I have…more in future, I promise. I focused the lazy way connecting camera to laptop via Helicon Remote – it could not be automated but I just clicked on ‘take picture’ and focused manually.
© Paul Harcourt Davies 2012
(NB. This article may be not be used in whole or in part without the express written permission of the author)