Petrographic microscope

A polarization microscope is a light microscope, the polarized light used for imaging. It is used for investigation of optically anisotropic ( birefringent ) objects. This may crystals or minerals with corresponding crystal lattice structure be ( intrinsic birefringence ) or isotropic materials, the mechanical forces act ( birefringence ). As a third group materials are to be mentioned, the birefringent due to their location and orientation properties develop ( form birefringence in biological or polymeric objects).

In addition to a "normal" light microscope containing a polarizing microscope, two polarization filters and a usually rotating stage. Sometimes known compensators are used to enhance the effects ( contrast ) or to analyze the strength of the birefringence.

History

In 1808 the French physicist Etienne Louis Malus discovered the refraction and the polarization of light. William Nicol invented in 1829 a prism for polarization, which was over 100 years indispensable component of the polarization microscope. Later, the Nicol prisms were replaced with cheaper polarizing filter.

The first complete polarization microscope was built in 1830 by Amici.

Rudolf Fuess built in Berlin in 1875, the first German polarizing microscope for petrographic purposes. This was described by Prof. Harry rose bush in the Yearbook of Mineralogy.

Design and rationale of the polarizing microscope

Polarizing microscopes typically operate in the transparent mode, although there are epi- polarization microscopes. In the transmitted-light polarization microscope is located beneath the specimen stage, a polarizing filter, polarizer, or also called a primary filter that polarizes the light from the linear light source of the microscope, that transmits only light vibrating in a vibration plane. This vibration direction is oriented parallel to the polarizer. Above the stage is a second polarization filter which is referred to as an analyzer or secondary filter, and is rotated with respect to the first by 90 °. The vibration direction of linearly polarized light before is characterized precisely oriented such that it is completely blocked by the analyzer. It has indeed no shares that vibrate in the Analysatorrichtung. Therefore, the image appears black. The arrangement of the primary and secondary filter is called a " crossed polarizers ".

Located on the stage between the two polarization filters a sample, the optical conditions are subject to change. Some chemical compounds, such as minerals have, under certain conditions, the property to rotate the plane of vibration of light, they are called birefringent or optically anisotropic. By changing the polarization plane is no longer relevant to the complete extinction - a part of the light passes through the analyzer and corresponding structures are visible. Also, it is possible to observe that occur due to interference colors. Optically isotropic materials, however, remain obscure.

Extinction rules

The extinction rules describe the conditions under which the image is dark:

Brightening and color interference

If an optically anisotropic material is oriented such that the allowed vibrational levels lie obliquely to the plane of polarization of the excitation light in the crystal, the light in the crystal is split into two beams with mutually perpendicular polarization planes split ( ordinary and extraordinary ray ). Of these, by the analyzer located in crossover position certain proportions are passed and there is a brightening of the image.

The typical polarization microscopy color images are created by interference. In a birefringent material, the light of the ordinary beam propagates at a different speed than the light of the extraordinary beam. When leaving the object, this results in a phase difference between the two beams as a function of the strength of the birefringence and the thickness of the object. As long as the vibrational levels of the two beams are perpendicular, they can not interfere with each other. The oscillating in Analysatorrichtung components are filtered only by the analyzer from two beams. These can either reinforce or cancel according to the rules of the interference. Since using white light, not all wavelengths are similar affected than excitation, it comes to the extinction of certain color ( wavelength ranges of light), and it can cause more vibrant and colorful images. The relationship between thickness, maximum birefringence and retardation of a crystal has put together in 1888 in a very concise form ( color scale according to Michel - Lévy ) A. Michel - Lévy.

Applications

The polarizing microscope is mainly used in mineralogy to the study of rock samples. In mineralogy, mostly thin sections are created that are irradiated. By analysis of the different optical properties and color as conclusions can be drawn about the composition of the rock sample.

Other areas of application are, for example, Texture studies of liquid crystals, analysis of the crystal growth, visualization of the mechanical stresses ( birefringence ), the visualization of the crystalline regions ( for example, spherulites ) in polymers, etc..