Polarization

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Revision as of 12:30, 20 October 2006 by Doos (talk | contribs) (Polarization by reflection)
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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

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.

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

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.

Polarization by double refraction


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