Why does the order of the interference color increase as the birefringence increases?

The order of the interference colors seen in the isochromes increases away from the melatope. As the thickness and birefringence of the crystal increase, the number of isochromes seen in the interference figure increases. A uniaxial interference figure can be used to easily determine the optic sign of a mineral.

What is the purpose of the Michel Levy chart?

Thus, the Michel-Lévy Birefringence Chart enables the microscopist to rapidly determine thickness, birefringence, or retardation, knowing the other two quantities. The original chart was used for aiding in the identification of minerals that make up rocks.

How do you use interference colors to determine birefringence?

Using interference colors to identify a mineral: Birefringence and interference colors vary for a given mineral depending on the direction the light follows through a grain. They also change as the microscope stage is rotated. If you are looking down an optic axis, the mineral appears to have no birefringence.

How do you calculate birefringence in a thin section?

Locate the interference color on the Michel-Levy chart, and find where it intersects the horizontal line for 30 micrometers. From this point, follow the diagonal line to the top of the chart to determine the birefringence value, and to see minerals with this birefringence.

What does the lower polarizer do?

The lower polarizer usually transmits polarized light virbrating in the front-back direction (or N-S direction). The petrographic microscope uses two polarizers. The upper polarizer is called the analyzer. The analyzer is mounted so that it can be inserted or removed with ease.

What causes high birefringence?

Modified proteins such as immunoglobulin light chains abnormally accumulate between cells, forming fibrils. Multiple folds of these fibers line up and take on a beta-pleated sheet conformation. Congo red dye intercalates between the folds and, when observed under polarized light, causes birefringence.

How does a birefringent material affect the state of Polarisation of a beam of linearly Polarised light?

A crystal of birefringent material affects light polarized in a particular direction differently from light polarized at 90 degrees to that direction; it refracts light polarized one way at a different angle than it refracts light polarized the other way.

What are interference Colours?

Definition. Liquitex® Interference Colours are colourless, transparent paints made from titanium coated mica flakes rather than traditional pigments. They are also known as “Opalescent Colours.” They change their colour (exhibiting a metallic look and colour shift) depending upon the viewing angle.

How do you determine retardation?

Retardation is a function of the mineral, its orientation, and its thickness. If the thickness is doubled, so is the retardation. Similarly, if a second crystal of the same mineral with the same orientation is placed on top of the crystal being studied, the retardation will increase.

What is a birefringent material?

Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive).

How can birefringence be used in mineral identification?

Many minerals exhibit birefringence, which is a directional dependence of the speed of light through the mineral. This phenomenon is used by petrologists to identify minerals in a thin section (≤ 1 mm) where a sample illuminated by polarized light is rotated under a microscope to reveal its birefringence.