The desktop polarizer is illuminated by a low-power light bulb mounted on the base.
When detecting the anisotropy of gemstones, the two polarization filters should be in an orthogonal position, so that no light can be seen or only a small amount of light can be seen before the detection. It is important to rotate the gemstone in all directions during detection.
1. Isotropic materials:
If a transparent material is dark regardless of its orientation between orthogonal polarizers, then it is isotropic. It is either amorphous or crystalline with a cubic crystal structure.
Using a polarizer to inspect uniaxial crystal materials
Fluorite beads are homogeneous and appear dark when rotated once under a polarizing mirror.
Planar polarized light enters isotropic materials through * polarization filters. Isotropic materials do not recombine incident light into two components. Polarized light continues to travel within its initial polarization plane and leaves the material. The polarized light enters the second polarization filter. Due to the orthogonality of the upper and lower polarization filters, the light entering * polarization filters is absorbed. Tuan'er, the material cannot be seen from afar.
2. Anisotropic materials:
When a material is able to transmit light and then darken after rotating on orthogonal polarizers, it is said to achieve extinction between orthogonal polarizers. If the extinction phenomenon appears to affect the entire material simultaneously and strictly occurs at 90 degree rotational intervals, then the material appears to be an anisotropic single crystal.
Using a polarizer to examine anisotropic materials
In the picture of this set of crystals, it is obvious that the inclusions in 1 and 3 have rotated 90 degrees, and this rotation is exactly 4 bright and 4 dark.
When either of these two vibration directions of the material is parallel to the vibration direction of the plane polarized light from * polarization filters, the polarized light will pass through the material and be absorbed by the second (upper) polarization filter.
At this rotational position, the material appears dark. When you rotate the material around your line of sight, its two vibration directions pass through every 90 degrees. Align with the polarization direction of the lower polarization filter once. So, by rotating it once, you can see four instances of extinction.
3. Polycrystalline materials
If a gemstone turns bright all round, it is said that the material is polycrystalline. For example, jade is composed of many small crystals. Each crystal produces birefringence. If not all of these crystals are parallel, then regardless of the orientation of the gemstones placed between the polarizing filters, there will always be some crystals that are not in the extinction position and will always appear bright.
Jade, including agate, is also polycrystalline and can effectively demonstrate this effect. It is composed of many extremely small crystals. It is said to be polycrystalline.
4. Abnormal extinction effect:
If a gemstone displays bright and dark areas, lines, bands, or cross shapes, it indicates that the material is under internal stress.
Glass is a single refracted object, but due to its uneven internal structure, it produces abnormal birefringence, which is identified as abnormal extinction. It is obvious that there is a black cross in the middle.
