Polarized light microscopy

Authors: Hans-Jürgen Schwarz, Anika Husen

back to Analysis of Salts

Abstract

The identification of salts with a polarized light microscope is briefly described including the advantages and disadvantages of the method.

Introduction

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Magnesium sulfate crystallization under crossed polars and red I

Polarized light microscopy [Wuelfert:1999]Title: Der Blick ins Bild
Author: Wülfert, Stefan
is used especially for the examination of anisotropic (birefringent) objects . This microscope differs from the ordinary microscopes by having polarizing filters added into the light path so that the object can be observed under polarized light. One filter, referred to as the polarizer, is located in the light path beneath the sample slide. Through it the object is illuminated with linearly polarized light. Another such filter, called the analyzer, is situated in the observation beam path allowing for the analysis of the linearly polarized light, modified by the object. When the polarizer and the analyzer are¨"crossed" (referred to as crossed polars) it means that there is a 90° difference in the vibration plane allowed through between them. When no samples is present, or a isotropic material is in the beam path, no light will come through.

In polarized light microscopy the direct (orthoscopic) or indirect (conoscopic) approach can be applied. The orthoscopic approach is equivalent to ordinary microscopy. When the analyzer is switched on anisotropic bodies appear. Depending on their orientation, thickness and the value of the birefringence anisotropic bodies appear in the interference color that corresponds to the path difference between ordinary and extraordinary rays.

A:In the orthoscopic light path (luke optical path) of older polarization microscopes the lens produces a vertically and laterally inverted intermediate picture of the thin section. This is further increased with the eyepiece (A-2). In modern polarization microscopes the object is located in the lower focal plane of the lens, causing it to be reproduced to infinity. The real intermediate image, visible through the eyepiece (A-1) is produced by an additional lens in the tube (tube lens). Through this display method a parallel beam path is created between tube lens and eyepiece, creating ideal conditions for an interference-free introduction of analyzers, compensators, or reflectors, and also allowing for a better aberration correction.
B: In the conoscopic beam path (pupillary light pathway), the reproduction of parallel light beams of the light cone takes place in the upper focal plane of the lens. The developing interference image (in the case of anisotropic crystals) can be magnified with an Amici- Bertrand lens. If no Amici-Bertrand lens is present, the interference image can also be seen through a diopter that can be inserted into the tube instead of the eyepiece.[Raith.etal:2009]Title: Leitfaden zur Dünnschliffmikroskopie
Author: Raith, Michael M.; Raase, Peter

Conoscopic approach: By switching on an additional lens (Amici-Bertrand lens) or by removal of an eyepiece, the back focal plane of the objective is pictured in the intermediate image plane, seen through the eyepiece. While in the orthoscopic approach every image point corresponds to an object point, in the conoscopic approach every image point corresponds to a parallel beam of light. Therefore the image gives information about the directionality of the birefringence (as far as it can be detected by the aperture). Consequently, this method allows to determine whether a crystal is optically uniaxial or biaxial and whether it is optically positive or negative. The light refraction of salt minerals can be estimated relatively easily, when the refraction index of the immersion medium or oil is known.

For a detailed description of microscopic mineral analysis see [Raith.etal:2009]Title: Leitfaden zur Dünnschliffmikroskopie
Author: Raith, Michael M.; Raase, Peter
.

Advantage:

Polarized light microscopy is a quick and convenient method for the identification of salts. The mineralogy and chemistry of salts is determined. Basic polarizing microscopes are portable and can be used in any location, hence sensitive salts can be identified on site.

Disadvantage:

Some salts are difficult to identify. Quantitative identification is not possible.

Literature

[McCrone.etal:1984]	McCrone, W. C.,; McCrone, L. B. ; Delly, J. G. (1984): Polarized light microscopy, McCrone Research Institute, Chicago, 9th ed. 1995
[Raith.etal:2012]	Raith, Michael M.; Raase, Peter; Reinhardt, Jürgen (2012): Guide to Thin Section Microscopy, Second Edition, online publication, Url,
[Wuelfert:1999]	Wülfert, Stefan (1999): Der Blick ins Bild, Ravensburger Buchverlag

the mathematical difference between the largest and smallest refractive index for an anisotropic mineral

The aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture also determines how many of the incoming rays are actually admitted and thus how much light reaches the image plane. The numerical aperture of an optical system is the product of the refractive index n and the sine function of the refraction angle α.