Difference between revisions of "Polarization"
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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:<br> | 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:<br> | ||
− | + | *Isotropic <br> | |
− | + | *Anisotropic<br> | |
− | + | *Anamolously Double Refractive or an<br> | |
− | + | *Anisotropic Aggregate<br> | |
The polarising filters in this instrument are made of a plastic with microscopically oriented crystals of quinine idosulphate. | The polarising filters in this instrument are made of a plastic with microscopically oriented crystals of quinine idosulphate. | ||
Revision as of 10:31, 26 February 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
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.
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.
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.
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
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
Expert
content needed
Retardation
content needed
Elipsoids
content needed