From The Gemology Project
Jump to: navigation, search

The following topics should be fully understood before studying this article.

Main article: polarization

Polarization is a concept that is fairly easy to comprehend. It plays a key role in many concepts and instruments that are important to gemologists. Understanding the basics of polarization is vital in your studies.

For this discussion we are going to regard light as traveling in waves, much like ocean waves. While ocean waves mainly go up and down, in light the waves can vibrate in any direction.
If we were to isolate a single lightwave, it could vibrate up and down (North-South), from the left to the right (East-West) or vibrate in any direction between those two.
Important to understand is that a single lightwave will vibrate only in one plane, like up and down or any other direction. No matter which direction that is. When this happens (and it always does), we say that the light wave is polarized.

Unfortunatly lightwaves never travel alone, instead light travels in endless bundles made of an infinite number of lightwaves. All of these lightwaves vibrate in different directions and the result is unpolarized light. Light coming from the sun or a lightbulb is always unpolarized.

Polarized light is light made up of lightwaves that all vibrate in the same direction
A single lightwave which vibrates North and South

To visualize this, you can imagine a skippy rope held by two people on opposite ends. If one of them would move his hand up and down fast, the rope will form waves going from one person to the other. The produced wave vibrates up and down (or better "North-South").
Similar if someone would move their hand left to right, the rope will vibrate like a snake (left to right).
You can move your hand even diagonal and the result will be a wave that vibrates diagonaly.

When we say that a wave vibrates in the up and down direction, those are actually 2 directions; one up and one down. So it is better to talk about the plane in which it vibrates. So the virbrational plane is in this case in the North-South directions. Or even better; the lightwave is polarized in the North-South plane.

As said before, light is made up from an infinite number of those polarized lightwaves and some travel in the same plane while many others vibrate in a random other plane. In the images below, a maximum of 4 lightwaves are illustrated and distinguishing them is already becoming difficult. Drawing a million of them would not even come close to reality, so you will have to use some imagination.

3-dimensional illustrations of polarized and unpolarized light
Polarized North South.jpg Polarized east west.jpg Unpolarized.jpg Unpolarized complex.jpg
A lightwave vibrating in the North-South plane (polarized) A lightwave vibrating in the East-West plane (polarized) Light made up from two lightwaves, each vibrating in a different plane (unpolarized) Light made up of four lightwaves, all vibrating in a different plane (unpolarized)

At this point it should be mentioned that the colors used in the illustrations are for illustration purposes only, so don't start thinking that all red light vibrates in the North-South plane. Some red lightwaves might, but not all do. It's random.

In gemology (and many other sciences) we sometimes want light to be polarized, that is we want light that only vibrates in a certain plane (like the North-South plane). In order to do that we must "block" all the light waves that do not vibrate in the desired direction.
For this purpose polarization filters were invented by Edwin Land (whom started the Polaroid Corporation).

two polarization filters in "crossed position"

A polarization filter lets only that portion of the light through that vibrates in a certain direction (plane). Those lightwaves could be any color, aslong as they are vibrating at exactly the right direction so the filter will allow them to pass through.

In the image to the left, two filters are illustrated. One that only lets through light that vibrates in the East-West plane (left) and the other that permits only light vibrating in the North-South plane to pass through (right).

If we were to put these two filters on top of eachother, you can imagine that no light is able to pass through (as illustrated in the overlap of the two filters). The first filter lets through only those lightwaves that travel in the East-West plane, so some light will pass through that and that light is now polarized in that plane. However when that, now polarized, light reaches the second filter it can not pass because it lets through only lightwaves that vibrate in the North-South directions.

Besides the polarization filters other materials can be used to polarize light. Even a table top can be used to do that, but that is an advanced topic.
When you get to the topic of birefringence, you will see that alot of gemstones also polarize light and the importance of this topic will become apparent.


Course home