Polarization is a concept that is fairly easy to comprehend. It plays a key role in many concepts that are important to gemologists. Understanding the basics of polarization is vital in your studies.
Light from the sun or a lightbulb is spherical, meaning that it transmits light in all directions.
If you were to follow one lightray in one direction (the direction of propagation), it would look like a circle with light being transmitted at an angle 90° to that direction. This is called transversal light. This transversal light is what is used as illustration for polarized light.
There are 3 causes of polarization, of which the first two are most relevant to gemology:
- Polarization by reflection
- Polarization by double refraction
- Polarization by scattering
A polarization filter lets through only light which vibrates in one direction (depending upon its orientation).
Polarizing filters are manufactured so that only a specific wave of transversal light can pass through the filter. This has a wide range of uses, from sunglasses to conoscopy and many other applications.
When you place a polarization filter in North-South (N-S) position, only waves vibrating in the E-W direction (or plane) are able to pass through the filter. As you rotate the filter to other positions, other orientations will pass through. This is what we call "plane polarized light", it only vibrates in one plane.
When two filters are used together and they are oriented in opposite directions, the end result is that no light can pass because one filter's behavior blocks the other. Let's say one filter is in North-South (N-S) position allowing only the E-W waves to pass, and the other is in East-West (E-W) position. When the light wave that passed through the first/N-S filter reaches the second/E-W filter, that second filter will let only waves in N-S position pass -- but no N-S waves are there, only E-W waves. Thus that second filter will block the E-W light wave, as can be seen on the image on the right.
Polarization by reflection
When light reaches an object, part of the light will be absorbed by the object (or refracted inside the object) while other parts of the light will be reflected by the object. This reflected light is partially to completely polarized, depending on the angle the light reaches the object. The polarization direction will be in the same direction as the surface of the object.
Sunlight striking the surface of water or a road are examples of reflected light. Most of the reflected light will be vibrating in East-West (E-W) direction, so polaroid sunglasses are orientated in North-South (N-S) direction to overcome the glare.
A Brewster Angle Meter makes use of this phenomenon.
Polarization by double refraction
When unpolarized light enters an anistropic gemstone at an angle other than the optic axis, that light will be split into two polarized rays, vibrating at right angles to each other. This is due to the double refraction properties of anistropic materials. On exiting the gemstone, these two polarized lights will combine again as unpolarized.
However, in some minerals (such as tourmaline), one of these polarized rays is completey absorbed by the mineral. This means that only one of the polarized rays will exit the mineral, and that ray will remain polarized. The phenomenon of absorbing light that travels in a certain direction is named selective absorption. The effect it causes is named pleochroism.
The first polaroid filters made were created from small grains of the mineral herapathite, which exhibits the same properties as tourmaline.
Another way of separating the polarized rays to create polarized light outside a gemstone is by making use of the critical angle of a substance. In gemology, a device that does this is called a Nicol prism, and many textbooks still mention the use of "crossed nicols". A piece of Iceland spar (calcite) is cut in two at a diagonal to the c-axis and is then cemented back together with Canada balsam, with nD = 1.537. Ordinary white light enters the Nicol prism and is split in two rays -- a slow ray (ordinary ray) and a fast ray (extra-ordinary ray). The ordinary ray will undergo Total Internal Reflection when it reaches the balsam. The extra-ordinary ray will pass through the balsam and leave the prism, polarized, on the other end.
- Introduction to Optical mineralogy 2004 - William D. Nesse ISBN 0195149106
- Gemmology 3rd edition (2005) - Peter G. Read ISBN 0750664495