Difference between revisions of "Polarization"

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'''''This page is due for a complete make-over''''' --[[User:Doos|Doos]] 06:27, 12 March 2006 (PST)
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Polarization is a concept that is easy to comprehent and plays a key role in many concepts that are important to us  
 
Polarization is a concept that is easy to comprehent and plays a key role in many concepts that are important to us  
 
gemologists.<br>
 
gemologists.<br>
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[[image:transversal.png|left|thumb|Transversal light]]
 
[[image:transversal.png|left|thumb|Transversal light]]
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===Polarization by reflection===
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[[image:polarizationbyreflection.png|thumb|left|180px|Polarization of transversal light by reflection]]
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===Polarization by double refraction===
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===Polarizing filters===
  
 
A polarization filter only lets through light which vibrates in one direction depending on its orientation.<br>
 
A polarization filter only lets through light which vibrates in one direction depending on its orientation.<br>

Revision as of 07:27, 12 March 2006

This page is due for a complete make-over --Doos 06:27, 12 March 2006 (PST)

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

Sphercial light

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.

Transversal light


Polarization by reflection

Polarization of transversal light by reflection


Polarization by double refraction


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.

Transversal light with light wave in N-S direction


This feature can be helpfull when combined with previous knowledge of double refraction.

Anisotropic minerals have the ability to split light into two rays.
This is due to their moluclar stacking and some other reasons.
As a result the two different rays travel through the gemstone at different velocities and they vibrate at right angles to eachother.

--illustration--

Polariscope

Polariscope.jpg

A polariscope uses polarized light for gem identification. It consists of two polarized filters, one on the top and one on the bottom of the instrument as seen in the above picture. Both the polariser and the analyser have their own vibrational planes. When the vibrational plane of the polarizer is at right angles to the vibrational direction of the analyser, the field between them remains dark. This position is known as the "crossed position". In this position, gems can be tested to determine if they are:

  • Isotropic
  • Anisotropic
  • Anamolously Double Refractive or an
  • Anisotropic Aggregate

The polarising filters in this instrument are made of a plastic with microscopically oriented crystals of quinine idosulphate.

Advanced

content needed

Conoscopy

Conoscopy Illustrated

Expert

content needed

Retardation

content needed

Elipsoids

content needed