Editing Mineralogy (section)

==Optical==
[[File:CSIRO ScienceImage 1483 Olivine Adcumulate.jpg|thumb|Photomicrograph of [[olivine]] adcumulate from the [[Archean|Archaean]] [[komatiite]] of [[Agnew, Western Australia]].]]
{{main article|Optical mineralogy}}

In addition to macroscopic properties such as colour or lustre, minerals have properties that require a polarizing microscope to observe.

===Transmitted light===
When light passes from air or a [[vacuum]] into a transparent crystal, some of it is [[Reflection (physics)|reflected]] at the surface and some [[Refraction|refracted]]. The latter is a bending of the light path that occurs because the [[speed of light]] changes as it goes into the crystal; [[Snell's law]] relates the bending [[angle]] to the [[Refractive index]], the ratio of speed in a vacuum to speed in the crystal. Crystals whose point symmetry group falls in the [[Cubic crystal system|cubic system]] are ''isotropic'': the index does not depend on direction. All other crystals are ''anisotropic'': light passing through them is broken up into two plane [[Polarization (waves)|polarized]] [[Ray (optics)|rays]] that travel at different speeds and refract at different angles.<ref name=Manual/>{{rp|289&ndash;291}}

A polarizing microscope is similar to an ordinary microscope, but it has two plane-polarized filters, a  (''[[polarizer]]'') below the sample and an analyzer above it, polarized perpendicular to each other. Light passes successively through the polarizer, the sample and the analyzer. If there is no sample, the analyzer blocks all the light from the polarizer. However, an anisotropic sample will generally change the polarization so some of the light can pass through. Thin sections and powders can be used as samples.<ref name=Manual/>{{rp|293&ndash;294}}

When an isotropic crystal is viewed, it appears dark because it does not change the polarization of the light. However, when it is [[Oil immersion|immersed in a calibrated liquid]] with a lower index of refraction and the microscope is thrown out of focus, a bright line called a ''[[Becke line test|Becke line]]'' appears around the perimeter of the crystal. By observing the presence or absence of such lines in liquids with different indices, the index of the crystal can be estimated, usually to within {{math|± 0.003}}.<ref name=Manual/>{{rp|294&ndash;295}}