The petrographic microscope

A petrographic microscope is used to observe a series of characteristics in a mineral which reflect its properties and allow us to identify it. The petrographic microscope is a compound microscope which can work with plane polarised light, meaning that it has some peculiarities. This is always done with transmitted plane polarised light, meaning that the polariser must be inserted. The type of illumination varies according to the feature to be studied, and may be orthoscopic (parallel, without the condenser) or conoscopic (convergent, with the condenser incorporated). The size of minerals that allows for optical identification is not samaller than 0.010 mm. Identification of cryptocrystalline and amorpous materials can be achieved using submicroscopic techniques such as a scanning electron microscope.

The petrographic microscope A petrographic microscope is used to observe a series of characteristics in a mineral which reflect its properties and allow us to identify it. The petrographic microscope is a compound microscope which can work with plane polarised light, meaning that it has some peculiarities. This is always done with transmitted plane polarised light, meaning that the polariser must be inserted. The type of illumination varies according to the feature to be studied, and may be orthoscopic (parallel, without the condenser) or conoscopic (convergent, with the condenser incorporated). The size of minerals that allows for optical identification is not samaller than 0.010 mm. Identification of cryptocrystalline and amorpous materials can be achieved using submicroscopic techniques such as a scanning electron microscope. Click on the button showing the feature you require: Stand This is the physical support to the other elements of the microscope. Attached to it are the mechanisms which move the stage and focus the sample. Other accessories are also joined to it by the same bracket as the stage, such as the condenser and the polariser. Illumination system(Hệ thông chiếu sáng) This is in the base of the stand at the foot of the microscope. In research microscopes, it is made up of a Light source (1), a set of Lenses (2) which allow a parallel beam of light to be obtained to avoid the loss of intensity by dispersion, an anti-thermic filter (3) which prevents the other elements from overheating, a set of chromatic filters (4) which allow the chromatic characteristics of the light to be modified, a mirror (5) to orient the light beam in the correct direction and an iris diaphragm (6) to regulate the light intensity and breadth of the beam. Polariser( Kính phân cực) This is situated immediately above the illumination system and below the condenser, although it is connected to the stage and the condenser by the same upright bracket. Its function is to convert the natural light from the illumination system into plane polarised light. The plane of vibration of the light in the polariser can be turned in some kinds of microscopes but in normal working conditions it is always fixed at 0°, often coinciding with what could be called the "East-West" direction. The polariser is always positioned in the pathways of the light rays for the study of any optical property. Polarised light is produced by the polariser and analyser, both of which in modern microscopes consist of a sheet of plastic (polaroid) which absobs all light except that vibrating in one direction. Older microscopes employed an ingeneous combination of calcite prism to produce polarised light (described first by W. Nicol and known as Nicol prisms). Condenser(Tụ sáng) Situated between the polariser and the object, it has a removable lens (1) in such a way that when it is fitted in the "ON" position it makes the light rays converge onto the thin section placed on the stage of the microscope. In this situation one speaks of convergent light or, more normally, conoscopic illumination. By contrast, when the convergent light lens is in the "OFF" position, the light rays no longer incide convergently but rather folow an approximately parallel path and all of them incide perpendicularly on the slide. In this situation one speaks of parallel light or, to be more correct, orthoscopic illumination. An iris diaphragm allows the illuminated area to be varied (known as the aperture diaphragm). For observing relief and Becke line it is usually necessary to partially close this diaphragm. Stage(Bàn soi) This serves as a support to the thin section (1) which is to be studied. It may be heightened or lowered (2) to allow the object to be focussed. It is round and can be rotated on a vertical axis which passes through its centre. It is graduated and on the outside it has a fixed goniometer (3) which allows the value of the angles turned to be measured accurately. Objetives These are the lens used for magnifying the specimen on the stage. Four or five are normally supplied (x4, x10, x25, x50). This allows a real and inverted image to be obtained of the object under examination. It lets the polarised light pass through without affecting the polarisation plane. Objectives are often mounted on a revolving objective holder (1), which allows quick and easy change of magnification. When the stage is rotated, the axis of rotation should coincide with the centre of the field of view (the axis of the microscope). This is achieved on some microscopes by adjusting a collar on the barrel of each objective (2) To centre a microscope, the point about which an object is seen to rotate when the stage is rotated must be brought to the centre of the cross-hairs by adjusting the centring screws. Focus El enfoque de la imagen en el microscopio se realiza separando el objeto a estudiar de los objetivos. Mediante unos anillos (1 y 2) se puede subir y bajar la platina para buscar el foco (en los microscopios antiguos la platina permanece fija, siendo los objetivos los que se desplazan). En este microscopio existe un amilla "macro" de desplazamiento brusco (1), para aproximar el enfoque, y uno, llamado "micro" (2), para ajustarlo. La operación de enfoque requiere seguir unos determinados pasos para realizarla con seguridad. El procedimiento correcto es el siguiente. Mirando por fuera del microscopio se lleva el objetivo junto a la preparación y mirando luego por el ocular se va separando lentamente hasta obtener la imagen. Si se busca el enfoque acercando el objetivo a la preparación se corre el riego de producir fracturas (en la preparación o, en lo que es mucho peor, en la lente frontal del objetivo) si nos pasamos del plano de foco. Slot for inserting compensators This is immediately below the analyser and its greatest dimension forms an angle of 45º with the direction of vibration of the Nicols (polariser and analyser). The main directions of vibration of the compensating plate and those of the polariser and analyser are situated at 45°. Compensators(Bù màu) These are plates of anisotropic substances, whose planes of vibration coincide with their two dimensions, length and width. Accessory plates consist of mineral sections of a thickness such that they produce a known amount of retardation. They may be wedge-shaped so that retardation (and thus colour) will depend on the thickness acting at any moment in time. The effects that these compensators introduce are superimposed on the effects that the minerals on the microscope stage introduce. They are used for studying interference figures and the retardation produced by mineral specimens. When required, they are inserted into the microscope tube in a slot between the objetives and the analyser. Analyser(Phân tích) This is above the objective, and is made up of a polarising plate, whose height may be adjusted at will, using a graduated dial (1). Unlike the polariser, the analyser does not always have a part to play in the passage of the light rays and can be installed or removed at will. It is used to study certain properties but is not necesary for others. The polarising plane is generally N-S, and it is always perpendicular to the polariser, such that if there is no object in the way, no light passes and Extinction occurs. Below it is a slot where compensating plates can be inserted. When only the polariser (1) is being used, a normal image is observed, but when the analyser is in place (2), an extinction of light occurs. If an anisotropic substance is in the light's path, the light splits into two rays which vibrate perpendicularly and do not necessarily coincide with the directions of the polariser or analyser. When these rays reach the analyser, two components come into being which vibrate, one on the plane of the analyser and the other on a perpendicular one. The former is responsible for the grain being seen, whilst the latter is annulled. A false colour appears, known as an interference colour (3). [...]... amplified by the ocular An alternative means of viewing interference figures is to remove the eyepiece and look down the microscope tube at the highe-power objetive lens, preferably wiyh the aid of a pin-hole stop inserted at the top of the tube Eyepieces(Thị kính) This is a system of lenses fitted to the top of the microscope and whose function is to form a virtual and amplified image from the real image... image created by the objective The eyepiece assembly contains two cross-hairs, and is slotted into the microscope tube so that the cross-hairs are orientated E-W and N-S, i.e parallel to the vibration directions of the polariser and analyser Most eyepieces have a magnification of x8 or x10 The focal plane of the new image is about 25cm from the upper lens, the normal distance of vision of the human eye... coherence to the soil samples so that they can be cut, lapped and polished In order to achieve the hardness required, the soil samples are vacuum packed in liquid polyester resin The resins harden as they polymerise, thus producing blocks which include the soil sample and conserve its natural structure unchanged These blocks can now be treated as if they were rock samples Preparation of sands The way to... particles Ray This is the rectilinear path followed by the wave (the path followed by the light) Wave length This is the distance between two points in phase with each other, which are those which are vibrating in the same manner, are at an equal distance from the state of rest and move in the same direction The different wave lengths are translated by the human eye into different colours: violet = 410... is found immediately below the ocular It may be incorporated (1) or removed (2) at will It can only be used when convergent light is used to observe the property called the Interference figure (3) The Bertrand lens magnifies and focusses interference figures but the Bertrand lens does not produce the interference figure but modifies the focal plane of the image formed by the objective to allow it to... mµ Frecuency This is the number of wave oscillations per second, which gives the wave velocity The part of the wave between two points in phase is called oscillation Velocity of propagation This is a characteristic of the medium in which light is propagated, and is determined by the Refractive index (n), or the relation between the velocity of propagation in a vacuum (c) and in the medium under consideration... prepare it The method depends on whether we are dealing with a coherent material (rock) or a loose material (soil and sand) Preparation of a rock Cut The first step is to cut the original rock in order to obtain a fragment with a flat surface similar in size to that of the preparation which we require Polish Once the flat surface is achieved, it must be polished to remove the roughness of the cut and... represented in diagram form along a rectilinear path that includes the state of vibration of the different points, successively reaching different displacements, from a state of rest The result of this vibration of adjacent points is the propagation of the wave We therefore have a very important point and one that we must keep in mind: that is, the directions of vibration and of propagation are perpendicular... Cementation After the surface has been polished, it is cemented onto a glass slide with resin or Canada Balsam Cut It is then recut, trying to make the second face parallel to the first and as thin as possible Lapping Next, the sample is lapped until it is only 50µm thick, so that with a final polish its thickness is between 20 and 30µm Cover The last stage is to cement a cover glass on top with the same material... las muestras de suelos (2) Oscillating theory of the light Light is a form of radiant energy, and although its precise nature involves very complex Physics theories, all the phenomena relating to minerals can be explained by exclusively considering the oscillating theory, i.e for our purposes, light is propagated as a consequence of a vibration of particles In the figure, a light wave is represented . electron microscope. Click on the button showing the feature you require: Stand This is the physical support to the other elements of the microscope. Attached to it are the mechanisms which move the. the stage is rotated, the axis of rotation should coincide with the centre of the field of view (the axis of the microscope) . This is achieved on some microscopes by adjusting a collar on the. The petrographic microscope A petrographic microscope is used to observe a series of characteristics in a mineral which reflect its properties and allow us to identify it. The petrographic microscope