I’ve always been fascinated by the mineralogy course when I was in university, especially when analyzing rocks in thin sections with old (very old) microscopes. The use of the famous “crossed Nicols” exploded the thin section into a thousand colours, and the minerals present showed incredible facets and behaviours.
Photo: Kevin Walsh on CC BY 2.0
The use of polarized light (light in which one of the two vector components has been removed and the resulting electromagnetic radiation is therefore in a single plane) is widely used in photography to remove reflections of light on specular surfaces (glass, water…); many sunglasses have filters of this kind to remove reflected light (especially partially polarized) from these surfaces. But if two polarizing filters are used, a condition is created whereby the light ray that passes through the first filter, if it encounters no other substance and passes through the second filter, will be more or less extinct until it is completely blocked if the two filters are rotated at an angle of 90°. If a substance is present between the two filters, a light ray will emerge that will be extinguished when the upper filter is rotated at an appropriate angle (also called the analyzer in polarimeters). This creates birefringence colours that take on different colours depending on the material. I highly recommend exploring this site https://www.microscopyu.com/techniques/polarized-light/polarized-light-microscopy and this page https://www.microscopyu.com/tutorials/polarizer-rotation-and-specimen-birefringence for more information.
So why not try analyzing amber in polarized light?
The base material is a light source that is as homogeneous as possible. For this purpose, I used the very useful LED panel from MJKZZ (https://www.mjkzz.de/collections/light-solutions/products/led-light-panel-pad?variant=30306597077107) which, with a useful surface area of 20x20cm, provides a powerful and homogeneous background light.
I then acquired two polarizing filters. Many can be found online. Given the dimensions of the light base, I acquired two filters of this type and size 20x20cm on Amazon (forgive me). One filter was cut to exact dimensions to be overlaid on the LED panel.
As seen in the photo below, the light is slightly attenuated since the filter has a grey colouration, but this does not affect the final result.
A glass plate is overlaid on the filter to protect against scratches and dust.
The elements to be acquired are positioned on top of the glass plate (it doesn’t matter in this case if they are left inside plastic containers). A cardboard box (again, forgive my lack of professionalism) will create a thickness between the subject (amber) and the second polarizing filter.
The second filter is inserted between two glass plates, also for the purpose of protecting it against scratches, and will cover everything. If oriented parallel to the first filter, no change in the colour of the amber will be noticed.
But if rotated 90° with respect to the first, the light will be extinguished and the birefringence colours of the amber will appear.
It is interesting to note, but this will be the subject of another article, that Myanmar and Dominican ambers behave in the same way, differentiating themselves from those of the Baltic Sea. Copal, on the other hand, shows no visible changes.
Below is a series of amber from Myanmar showing their wonderful colours.