Pixiq

Hidden colors revealed by crossed polarizers

Finding the 'art' hidden in nature has been a lifelong passion of mine. Did you know that 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). It is no easy task...

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.

Find out more about how these images were obtained.

I have covered polarisation in a previous Pixiq blog posts but this is not a post about the physics of materials 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.

The two images below show how the rock sample appears with just one polariser and then with the second added

ABOVE- a rock section with plane-polarized light. BELOW crossed polarisers reveal hidden colours

©  Paul Harcourt Davies 2012

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