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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

Having completed a survey of the characters employed in mineral identification when the polarizing microscope is used without either nicol, the attention of the viewer may be directed to the characters to be observed when the lower nicol or the polarizer is inserted in the position in the axis of the polarizing microscope. The upper nicol or analyzer on the other hand is still swung aside.

 

            In order to understand the phenomena, it is necessary first to realize some elementary facts as to the nature of light and the construction of the Nicol’s prism. It is important to note that as light may be transmitted through what is known as a vacuum, it is necessary to assume the existence of a substance pervading all space, by means of which the transmission is effected. This substance is known as the ether. It is quite certain that light is transmitted as a consequence of a series of vibrations which takes place at right angles to the direction in which light is traveling, and the theory formerly accepted was that the ether particles themselves performed these vibrations to and fro without undergoing any movement along the line of propagation of the ray. The theory has now been considerably modified, and a theory of oscillation, not of the particles themselves, but of their electrical condition is now held to account for the facts more satisfactorily. But these oscillations or vibrations certainly do take place, and they take place at right angles to the propagation of the light. A ray of ordinary light performs its vibrations in all directions possible subject to this limitation, that is, in all the directions perpendicular to the direction of transmission. But light, which has passed through a nicol of the polarizing microscope, performs all its vibrations, not merely perpendicular to this direction, but parallel to a definite line, which fulfils this condition. Such light, performing its vibrations in one plane only, is said to be polarized, and we have now to discuss something about the construction of a nicol in order that we may realize the conditions of the investigation using the polarizing microscopes for geologists.

 

            The nicol is made of a clear variety of calcite known as Iceland Spar, a substance possessing to a remarkable degree the power of splitting a ray of entering light into two. This property is known as double refraction. The mineral has an extremely low good cleavage in three directions that can be found clearly exhibited under geological polarizing microscopes, and, split along these cleavages, it yields a fragment of the form known as the rhombohedron. Light entering to such a fragment perpendicularly to one of the cleavage planes is split into two rays, and, in consequence, any object viewed along such a perpendicular is doubled. Two images reach the eye through the lenses of the petrographic polarizing microscope. The most satisfactory demonstration of this fact is obtained by resting the fragment on a sheet of white paper on which a small blot of black ink has been made. Seen from above, such spot is doubled. When the fragment is rotated, it can be found that one of the images remains stationary, while the path of the other is the circumference of a circle with the stationary image as the center. The stationary image behaves just as if the crystal were a slab of glass. It is therefore said to be produce by the ordinary ray. The ray producing the movable image, on the other hand, does not obey the ordinary laws of refraction. To anyone who has studied only singly refracting substances, its behavior appears to be quite extraordinary. It is therefore known as the extraordinary ray.

 

            By looking at the surface obliquely using the polarized light microscope, we can see that the ordinary image appears to occupy a higher position inside the fragment than does the extraordinary image. In other words, the density of the crystal for the ordinary ray is greater than for the extraordinary ray. Or, the velocity of the extraordinary ray is greater than that of the ordinary ray. The fact that two such rays traveling in a doubly refracting crystal have different velocities is extremely important to a beginner who wishes to understand what follows, and this experimental demonstration of the fact should help materially to fix the ideas.

 

            We have seen, then, that a ray of light incident normally to a face of such a rhombohedron is split into two, each ray having its own path and a velocity, which depends on the nature of the crystal. Still further information may be obtained if the two spots are examined through a nicol. The nicol of the polarizing microscope may, for the present, be considered as an instrument capable of transmitting only that light which vibrates in a direction known as the principal section. The nicol is rotated on a vertical axis, it is seen that the two spots appear and disappear alternately when closely evaluated on the lenses of the polarizing microscope for geologists, and that a rotation of 90 degrees is necessary to produce this alternation. Granting our assumption as to the direction of the vibrations transmitted by the nicol, it is easily demonstrated that each ray emerging from the cleavage fragment is polarized or it performs its vibrations parallel to one line only. And the two vibration directions, that of the ordinary ray and that of the extraordinary ray, are at right angles. Clearly, then, if it is possible to prevent the emergence of one of these rays from the Iceland Spar, an efficient instrument for the production of the polarized light may be obtained. Such elimination is effected by the nicol. Before its operation can be understood, however, an individual who do the evaluation must realize what is meant by total internal reflection.



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Wednesday, February 13th, 2008 at 3:46 am
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Characters of Minerals in Polarized Light
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