Difference between revisions of "Polarization"

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Another way of seperating the polarized rays to create polarized light outside the gemstone is by making use of the [[critical angle]] of a substance. In gemology we know such a device as a Nicol prism and many textbooks still mention the use of "crossed nicols".<br />
 
Another way of seperating the polarized rays to create polarized light outside the gemstone is by making use of the [[critical angle]] of a substance. In gemology we know such a device as a Nicol prism and many textbooks still mention the use of "crossed nicols".<br />
 
A piece of Iceland Spar ([[Calcite]]) is cut in two at a diagonal to the c-axis and then cemented back together with a balsam with n<sub>D</sub> = 1.537. Ordinary white light enters the Nicol prism and is split in two rays, a slow ray (ordinary ray) and a fast ray (extra-ordinary ray). The ordinary ray will undergo [[Brilliance|Total internal Refrlection]] when it reaches the balsam, while the extra-ordinary ray will pass through the balsam and leaves the prism, polarized, on the other end.
 
A piece of Iceland Spar ([[Calcite]]) is cut in two at a diagonal to the c-axis and then cemented back together with a balsam with n<sub>D</sub> = 1.537. Ordinary white light enters the Nicol prism and is split in two rays, a slow ray (ordinary ray) and a fast ray (extra-ordinary ray). The ordinary ray will undergo [[Brilliance|Total internal Refrlection]] when it reaches the balsam, while the extra-ordinary ray will pass through the balsam and leaves the prism, polarized, on the other end.
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==Related topics==
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* [[Brilliance]]
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* [[Reflection effects]]
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* [[Polariscope]]
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==Sources==
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* ''Introduction to Optical mineralogy'' 2004 - William D. Nesse

Revision as of 13:24, 20 October 2006

Polarization is a concept that is easy to comprehent and plays a key role in many concepts that are important to us gemologists.
Understanding the basics of it is vital in your studies.

Basics

Transversal light with light wave in N-S direction


Light from the sun or a lightbulb is spherical. Meaning that it transmits light in all directions.
If you would follow one lightray in one direction (the direction of propogandation) it can be seen as a circle with light being transmitted at a 90° angle to that direction. Something which we call transversal light.

It is this transversal light that we use as illustration for polarized light.

There are 3 causes for polarization of which only the first two are relevant to gemology:

  • Polarization by reflection
  • Polarization by double refraction
  • Polarization by scattering


Polarizing filters

A polarization filter only lets through light which vibrates in one direction depending on its orientation.
Imagine a lightsource traveling towards a wall. When it reaches the wall no light can pass through it. If you would cut horizontal slices out of the wall the effect would be a Venetian blinds effect on the other side of the wall.

The same happens with polarizing filters. They are manufactured in a way that only a specific wave of transversal light can pass through the filter.This has wide use, from sunglasses to conoscopy and many more.

Crossed polarization filters


If you would position the polarisation filter in North-South position only waves in the N-S position are able to pass through the filter. On rotation of the filter other orientations will pass through.
This is what we call "plane polarized light".

When two filters are used and they are orientated in opposite directions, no light can pass at the end because one is in N-S position (letting only the N-S waves to pass) and the other is in East-West. When that light wave reaches the second filter that 2nd filter will only let waves in East-West position pass. Which is not there. Thus it will block the remaining North-South lightwave as can be seen on the image on the right.

Advanced

Polarization by reflection

Polarization of transversal light by reflection

When light reaches an object, part of the light will be absorbed by the object (or refracted inside the object) while other parts of the light will be reflected by the object.
This reflected light is partially to completely polarized, depending on the angle the light reaches the object.

Sunlight striking the surface of water or an a road are examples of this and people wear sunglasses to block most of that reflected light. The polarization direction will be in the same direction as the surface of the object, thus in the case of sunlight reaching water most of the reflected light will be vibrating in East-West direction and the polaroid sunglasses are therefore orientated in North-South direction to overcome the glare.
A Brewster Angle Meter makes use of this phenomenon.

Polarization by double refraction

When unpolarized light enters an anistropic gemstone at an angle other than the optic axis, that light will be split in two polarized rays each vibrating at right angles to eachother. This is due to the double refraction properties of anistropic materials. On exciting the gemstone, these two polarized lights will combine again and will become unpolarized.
However in some minerals (as Tourmaline) one of these polarized rays is completey absorbed by the mineral and only one of the polarized rays will exit the mineral and will remain polarized. The phenomenon of absorbing light that travels in a certain direction is named selective absorption and the effect that it causes is named pleochroism.
The first polaroid filters were created from small grains of the mineral Herapathite which exhibits the same properties as Tourmaline.

Another way of seperating the polarized rays to create polarized light outside the gemstone is by making use of the critical angle of a substance. In gemology we know such a device as a Nicol prism and many textbooks still mention the use of "crossed nicols".
A piece of Iceland Spar (Calcite) is cut in two at a diagonal to the c-axis and then cemented back together with a balsam with nD = 1.537. Ordinary white light enters the Nicol prism and is split in two rays, a slow ray (ordinary ray) and a fast ray (extra-ordinary ray). The ordinary ray will undergo Total internal Refrlection when it reaches the balsam, while the extra-ordinary ray will pass through the balsam and leaves the prism, polarized, on the other end.

Related topics

Sources

  • Introduction to Optical mineralogy 2004 - William D. Nesse