Polarizers; polarizing microscope

Polarizers; polarizing microscope

Light, which can be obtained by reflection from smooth, flat surfaces, and by passing non-polarized light through the optically anisotropic crystal, it is only partially polarized. Polarizing prisms are commonly used to obtain fully polarized light, constructed of transparent calcite crystals. In the prisms of these, called Nicola prisms or nickels for short, one of the polarized rays is removed. Microscope, with built-in nickels is called a polarizing microscope, also called a petrographic or mineralogical microscope, because it is sometimes used to study rocks and minerals.

Polarizing microscope, just like an ordinary microscope, has three lens systems, i.e.. lens placed at the bottom of the tube, an eyepiece with a cross of perpendicular threads and an illuminator under the microscope table inserted from above. Lenses and glasses are interchangeable and their appropriate selection determines the degree of magnification of the viewed object. The illuminator is used to focus light rays on the observed object.

Polarizing microscope: a - general view, b - construction scheme.

The polarizing microscope differs from the ordinary one mainly in the fact that it is fitted with two nickels. Under the rotating round microscope stage, equipped with a scale for measuring angles, there is a lower nikol - a polarizer. Between the lens and the eyepiece there is a top nickol - analyzer. Nikola's position in relation to each other is so chosen, that the light vibrations transmitted by the lower nickol are stopped by the upper nickol (never crossed). The analyzer is retractable; part of the microscopic observation of rocks and minerals is carried out without the analyzer, using only the polarizer.

With a flat-concave mirror, placed under the polarizer and illuminator, a beam of natural or artificial light is inserted into the microscope, to - after passing through nikol bottom, microscope specimen placed in the center of the stage, lens and eyepiece - it reached the observer's eye. Using two screws, one of which allows for a greater shift of the tube, and the other (micrometric) for fine adjustment, the field of view is sharp, magnified image of the observed specimen. These preliminary preparations are performed without the analyzer.

The microscope lens should be so centered, so that the specimen point observed in the center of the field of view, i.e.. at the intersection of the cross strands of the eyepiece, it did not shift during the rotation of the microscope stage. When the upper nickol slides into the microscope tube, a complete obscuration of the field of view can be observed, as long as the main sections of both nickels are exactly perpendicular. If there is no complete blackout, you should turn the bottom nikol a bit like this, that the crossing of nikola is complete. The plane of light vibrations in both nickels should match the directions of the eyepiece threads.

Dimming the light, also known as fading out or disappearing, occurs between crossed nikols then, when light passes directly through the air from the polarizer to the analyzer, or if there is a transparent amorphous body in the path of polarized light produced in lower nickol, e.g.. glass or transparent mineral belonging to the regular system, e.g.. diamond or fluorite. These bodies, as well as the glass of the illuminator and lens, they behave indifferently to polarized light.

The optically isotropic crystals placed between crossed nickels darken the field of view and do not change during the rotation of the microscope table. On the other hand, optically anisotropic crystals behave quite differently. During 360 ° rotation of the microscope stage, with an optically anisotropic crystal between the crossed nickels, the light is dimmed four times. This will happen in such positions, in which the vibrations of light in the crystal plate will follow the direction of vibrations in nickol and parallel to the directions of the spider's threads of the eyepiece. The light vibrations transmitted through the crystal plate will then be stopped by the analyzer (upper nikol), transmitting only vibrations in a plane perpendicular to the plane of light vibrations in the polarizer. In this position, the light will dim, analogous to that occurring in optically isotropic bodies.

Dimming the light, which takes place in optically anisotropic crystals between crossed nickels, can be simple, oblique or symmetrical to rectilinear crystallographic directions, like the edges, traces of cleavage or planes of twin adhesions.

Simple dimming of the light takes place with the arrangement of the directions of vibrations in nickels in accordance with the crystallographic direction. The cross of the spider strands is parallel to the crystallographic direction. This kind of dimming occurs in the crystals of a tetragonal system, hexagonal and trigonal and rhombic on the walls of columns, and also on some faces of the crystals of a monoclinic system.

The oblique dimming of the light differs from the simple one in that, that the directions of vibrations in the crystal form an angle with the crystallographic directions. Light dimming angle, characteristic for a given face of the tested crystal, allows the identification of many minerals. The oblique dimming of the light occurs on all crystal faces of a triclinic system and on most crystal faces of a monoclinic system.

Symmetrical light dimming is about this, that the directions of vibrations in the crystal follow the direction of the bisector of the angle between the straight lines that limit the crystal. This type of light dimming occurs on the walls of the hexagonal pyramids, tetragonal and trigonal. They can be considered as a special case of simple dimming (relative to the plane of symmetry of the mineral).

Wavy dimming of the light is different from the usual (simple), oblique or symmetrical, that it does not occur simultaneously throughout the mineral. When certain parts of the mineral are already dark, others are light or gray. This phenomenon is common in quartz, especially as a component of metamorphic rocks, which have been subject to strong directional pressure.

The twin crystals can be easily recognized between the crossed nickels. A crystal that seems to be single in ordinary light turns out to be composed of parts, which show dimming in different positions. In this way, both single twins can be found, and multiple.