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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

Polarization of light may come in various ways and this is one of the areas concerned in optical mineralogy. A light can be polarized by selective absorption, double refraction, reflection, and scattering.

The basis of polarization with polarizing films is provided by the selective absorption. Tourmaline and a variety of other anisotropic minerals posses the property of being strongly absorbing light vibrating in one direction and transmitting light vibrating at right angles more easily. This property of minerals is called pleochroism. The unpolarized light split into two plane polarized rays that vibrate perpendicular to each other when this unpolarized light enters one of these anisotropic minerals in the proper direction. Each of the two rays gets almost about half of the light energy. One of the rays is absorbed and the other passes through if the material is thick enough and is strongly pleochroic like tourmaline. Upon leaving the material, the ray that passes through retains its polarization.

In 1928, Edwin Land developed the original polarizing film. This film took advantage of the strong pleochroism exhibited by a wonderful mineral called herapathite. This mineral herapathite forms long slender crystals that are usually wonderfully exhibiting interesting microscope images under polarizing microscopes. Millions of tiny grains of mineral herapathite were embedded in plastic. These are then aligned by extrusion of the plastic through a narrow slit. The resulting plastic sheets behaved as though they were large flat sheets of mineral herapathite. In terms of polarizing the light that passed through them, the crystals actually did a credible job. Nowadays, the more recent developments have dispensed with the herapathite crystals and they have relied more on long hydrocarbon molecules in the plastic. The resulting medium is a sheet of polarizing film with substantially better optical properties. All modern petrographic polarizing microscopes nowadays use sheets of polarizing film to provide polarized light.

Another means of producing polarized light is dependent on the double refraction of anisotropic minerals. The Nicol prism is the most commonly used devise that meets up this principle on optical miceralogy. Nicol prism is contrasted of clear calcite. In preparation, this prism is made up of a crystal of calcite that is cut on the diagonal and then glued back together with the use of a balsam. A balsam has an index of refraction of 1.537. When light pass through the calcite, it split into two plane polarized rays, namely ray e and ray w, which have different velocities. These two rays vibrate at right angles to each other. Each ray can be assigned an index of refraction because they have different velocities. It turns out that that the index of the first ray is less than that of the balsam, while the other ray has greater index than that of the balsam. The cut through the calcite crystal is oriented for the purpose that the ray w would strike the boundary between the balsam cement and the calcite at greater than the critical angle. Hence, ray w is internally reflected and absorbed by black paint on the side of the prism. The ray e on the other hand enters the balsam. Then it is transmitted through the prism to emerge at the top, now as plane polarized light. The most preferred means of polarizing light is through the use of a Nicol prism in petrographic polarizing microscopes until replaced by polarizing film.

The unpolarized light that strikes a smooth surface like a piece of glass, the light is then reflected and is polarized so that its vibration direction is parallel to the reflecting smooth surface. Unless the angle between the reflected and refracted rays is 90 degree, the reflected light is not completely plane polarized. A Brewster’s angle is the angle of incidence needed to produce the 90 degree angle between the reflected and refracted rays. The Brewster’s angle can be derived from the Snell’s law if the indices of refraction are known. The refracted light is polarized so that it vibrates at right angles to the reflected light. But then it usually includes a substantial component of light that vibrates parallel to the surface, that is why it is not considered as a completely plane polarized light.

By looking through the film at the glare reflected from a smooth horizontal surface like a tabletop, the polarization direction of the polarizing film can be easily identified. The polarizing film will transmit a maximum amount of glare when its polarization direction also is horizontal because the polarization of the reflected glare is horizontal. The film will absorb the glare if the polarizing film is rotated so that its polarization direction is vertical. The sunglasses that use polarizing film have it oriented with the polarization direction vertical. This is for the purpose that it absorbs the reflected glare coming from horizontal surfaces.

Polarization by scattering on the other hand is of no particular concern on optical mineralogy. But then it is responsible for the blue color of the sky and also of the brilliant colors of the sunset. A certain amount of the light is scattered by the dust particles in the air and by the air molecules themselves when light passes through the air. The light that is scattered is polarized so that it vibrates in plane in a plane at right angles to the original path of the light before scattering. By looking at the bluest part of the sky with a piece of polarizing film whose vibration direction is known, this polarization can be demonstrated. The wavelengths that are most strongly scattered are very much dependent on the size of the scattering particles. It turns out that blue light is most strongly scattered for the atmosphere. Hence, the sky looks blue from the blue light that is scattered down to us from light passing through the upper reaches of the atmosphere. Sunrises and sunsets take on colors in the red, orange, and yellow end of the spectrum. This is because the light reaching us directly from the sun has had much of the blue end of the spectrum scattered away as it passes at a low angle through the atmosphere. Another very effective means of scattering the light that may also produce vivid sunrises and sunsets is the volcanic dust that are blown into the upper parts of the atmosphere.



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Wednesday, February 27th, 2008 at 3:50 am
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Optical Mineralogy
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Click Here For Best Selection Of High Quality Polarizing Microscope