Petrographic Microscope Parts And Functions Pdf
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Microscopes are generally made up of structural parts for holding and supporting the microscope and its components and the optical parts which are used for magnification and viewing of the specimen images. This description defines the parts of a microscope and the functions they perform to enable the visualization of specimens.
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What does a microscope look like? Although all petrographic microscopes have common parts and functions, there are many brands of microscope, each of which may look different. If petrographic microscopes are maintained and handled carefully, they can last for literally generations of students. The microscopes in your classroom or the laboratory could be new, a few years old, or decades old, and have different design aspects from younger or older microscopes.
If all microscopes look somewhat different from each other, are the examples shown here relevant to your microscope? They should be! It turns out that even though various microscopes might look different at first glance, because all petrographic microscopes will have common functions, the overall design and structure of microscopes is similar. Even if the analyzer on your microscope looks different from mine, they should be in a similar location on both microscopes and should function in a similar way.
The parts of the microscope that are located above the base but below the stage are called the substage assembly. The substage assembly includes the condenser, the lower polarizer, light filters, and diaphragms.
The polarizers in petrographic microscopes are plane (linearly) polarized. See section 2.3 for more information about how polarizers work. There is a polarizer in the substage assembly, and a second polarizer called the analyzer between the objectives and the Bertrand lens.
Typically the lower polarizer and analyzer are oriented at 90 degrees to each other, and oriented N-S and E-W relative to the base of the microscope. Polarization directions are marked either on the base of the microscope, or on the polarizers themselves. In some microscopes, the analyzer and the lower polarizer can be rotated (Figure 2.4.13) for custom measurements. This is not necessary for standard petrographic analyses.
A petrographic microscope, also called a polarizing microscope, is best described as a compound, transmitted-light microscope to which components have been added to enable the determination of the optical properties of translucent substances. The designation of the microscope as a compound microscope indicates that it has an ocular that focuses on a virtual image of the subject produced in the tube of the microscope by the objective lens.
The professional petrographic microscope has a substage condenser that can be centered and focused. The substage has field and aperture diaphragms. The polarizing components are the upper and lower polarizing devices, the Bertrand lens and its mounting (between the upper polarizing device and the ocular), an accessory flip-in lens for convergent light mounted as the top element of the condenser, and a graduated rotating stage with a removable click stop that can be activated to indicate a 45-degree rotation from any selected direction. The focus knob(s) and the stand are graduated to permit the determination of thickness by differential focusing.Figure 147 shows a petrographic microscope
In modern petrographic microscopes, the nicols are polarizing plates fabricated in much the same way as are the lenses in polarized sunglasses. If two polarizing plates of sufficient thickness and quality are superimposed with their polarization directions at right angles to each other (thus,crossed nicols), no light can penetrate the pair. This is because the first polarizing plate excludes all light that is polarized perpendicular to the direction of the plate polarization, concomitantly polarizing the remaining light parallel with the plate polarization, and then the second plate does likewise. Together, all light is excluded. An indication of this effect can be observed by looking through two polarized sunglass lenses superimposed 90 degrees to each other. Polarized sunglasses work because all reflected light is (at least partially) polarized parallel with the substance from which it is reflected and the polarizing material prevents the passage of rays polarized in a horizontal direction, such as that reflected from puddles, snow, automobile surfaces, and pavement. The effect may be observed if you look through a polarized lens at a patch of reflected glare while turning the lens to various orientations relative to the polarization of the glare. (The sky is polarized by reflection from the molecules of the atmosphere.)
A properly adjusted petrographic microscope with nicols of sufficient quality allows no light discernable to the eye to penetrate the upper and lower polarizing devices when their polarization directions are 90 degrees to each other and there is no birefringent substance between them. However, if a birefringent substance (such as a crumpled piece of cellophane) is placed between two polarizing plates that are positioned with their polarization directions 90 degrees to each other, the birefringence of the inner substance polarizes the light that travels through the first polarizing plate in directions parallel to the optical directions of the substance and the optical system will transmit light. The intensity and color of this transmitted light are controlled by thebirefringence, optical orientation, and thickness of the interior substance. The lower and upper nicols, or polarizing plates (called the polarizer and analyzer, respectively), of the petrographic microscope act as do the plates of polarized material. If both nicols are in the optical path and are oriented at right angles to each other and a birefringent specimen material is on the stage, the amount of the birefringence and many other optical properties may be determined.
In the petrographic microscope, the light is collimated by the condenser into a bundle of beams,all parallel to the optical aids of the microscope. The specimens examined are transparent to translucent thin sections or grain mounts of the material under study (see sections 5.3 and 5.4).The light beams are polarized in one direction (by the polarizer) before the light reaches the specimen. This light is called plane polarized light.
When both nicols are in use in this standard orientation, the object on the stage is said to be viewed with crossed nicols or polars. In addition, most petrographic microscopes are equipped with a slot at 45 degrees to the main polarization directions and various retardation plates that can be used in this slot in the determination of a number of optical properties. The most common and most useful of these plates is the 1/4 (one-quarter) wave plate [is correct], or gypsum plate.
The uses of the petrographic microscope include identifying translucent substances by means of their optical properties and by reference to the various charts and tables in the literature (Bloss, 1961; Kerr, 1959; Larsen and Berman, 1964; Rogers and Kerr, 1942). Thus, the composition and identity of these substances and the relationships between the various phases of the material under study may be discovered. From these data, facts concerning the history and the method of formation of the subject material can be deduced. The optical properties of substances given in texts, charts, and graphs have generally been accurately determined by the use of a universal stage on which the substance can be oriented at any desired angle to the optical axis of the microscope and the plane of rotation. In this manner, the optical properties have been determined for various substances. The concrete petrographer does not usually try to attain this degree of accuracy.
Courses on the use of the petrographic microscope are available at most colleges and universities that have departments of mineralogy or geology, in most departments of materials engineering,and in some departments of chemistry. The textbooks available on this subject vary widely in their emphasis. Certain texts concentrate on the theories of the behavior of light in various types of crystal structures, use of the Bertrand lens, and various optical axis figures. Some are oriented toward identifying and naming the minerals, others concentrate on teaching recognition of individual types of rock.
The identification of aggregate minerals and rocks and concrete reaction products may usually be most efficiently accomplished by knowing which mineral substances are likely; noting the outward physical properties, color, cleavage, and hardness either in a hand specimen or with the stereomicroscope; and using the petrographic microscope to observe the general appearance in polarized light to determine the approximate birefringence, indices of refraction, and some of the other optical properties. The procedures involved include determining some of the following optical properties, listed in order of most common usage:
A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks. The method is called "polarized light microscopy" (PLM).
Petrographic microscopes are constructed with optical parts that do not add unwanted polarizing effects due to strained glass, or polarization by reflection in prisms and mirrors. These special parts add to the cost and complexity of the microscope. However, a "simple polarizing" microscope is easily made by adding inexpensive polarizing filters to a standard biological microscope, often with one in a filter holder beneath the condenser, and a second inserted beneath the head or eyepiece. These can be sufficient for many non-quantitative purposes. 2b1af7f3a8