polarization of light

[bwanaaa]

I continue to be befuddled by the actual events that occur when light is polarized when it passes through a polarizer. What exactly does the chaotic array of molecules in the filter do to align photons? I can see how a crystal might do this but polarization is also a property of organic molecules. Anyone? Cyclowizard, dont you work in an organic polymer lab? Is this a property you care about?

 

[stevf]

it has been ages since I dealt with polarizers but i think it has to do with the fact that organic molecules are often a very long chain of carbon atoms with hydrogen and other atoms coming off them so they are not in a chaotic array but nicely aligned parallel to each other - at least the ones they would use in a polarizing filter

 

[Rockinacoustic]

From my understanding, the specific rotation of the polarized light is the result of photons interacting with the electron clouds of the molecules. This is why chiral molecules (or those that cannot be superimposed on their mirror images) are commonly referred to as 'R' or 'S' enantiomers. They are the same molecule, but are oriented in space differently, and thus they have opposite but equal specific rotations. (if your unfamiliar with organic chemistry sorry for the confusion ).

I came across This article which gives a good idea of what's going on.

However, I've only briefly studied the theory of plane polarized light in relation to stereochemistry in my Organic Chem class, so perhaps somebody else can explain the physics of the phenomenon.

 

[CycloWizard]

This is something that I probably should know, especially since our lab develops optical polymer materials. However, we generate our optical properties by adding nanoparticles to our polymer systems, so I know very little about how a bulk polymer material generates its optical properties. From the little I do know, I think Rockinacoustic might be right and the chirality of the polymers might be important.

 

[bwanaaa]

the curious fact is that the millions of molecules are randomly distributed in all possible orientations. The incident photon is then affected by the electron cloud the same way, no matter where it hits the cloud. Considering the electron cloud is asymmetric and can be quite complicated, it is counterintuitive that the effect would be the same.

you might expect different amount of rotation depending on where in the cloud the photon impacts. but i suppose it's like the following analogy-consider a spinning billiard ball that impacts on an irregular surface. Although the direction of its reflection is determined by the angle of incidence with the surface, the amount of change in its spin will be the same no matter where it impacts. Only by changing the character of the surface will the change in spin be affected.

 

[Nathelion]

The quick and dirty explanation I've heard goes something like this:

Imagine a large number of thin parallel conducting bars. For example, lets orient them vertically. Now light can be described as two orthogonal electrical waves (ignore the magnetic part for now). As the E-field propagates through this grating, the vertical component moves electrons up and down in the grating and is essentially absorbed this way. There are no electrons that can move in the horizontal direction, however, so the horizontal component is not absorbed. This is obviously a simplified model, but I think that's the gist of it.

 

[dkozloski]

Originally posted by: Nathelion
The quick and dirty explanation I've heard goes something like this:

Imagine a large number of thin parallel conducting bars. For example, lets orient them vertically. Now light can be described as two orthogonal electrical waves (ignore the magnetic part for now). As the E-field propagates through this grating, the vertical component moves electrons up and down in the grating and is essentially absorbed this way. There are no electrons that can move in the horizontal direction, however, so the horizontal component is not absorbed. This is obviously a simplified model, but I think that's the gist of it.

That's the way I heard it.

 

[PolymerTim]

I'm also a polymer guy and have little physics understanding, but I may be able to shed a little light on the subject <pun intended>. It looks like most of the answer is here in the posts above with just a little clarification needed. First, here's a couple of good links I used to help me brush up on the subject. I use polarizers in a microscope all the time so I guess it would be nice to know how they work.

http://en.wikipedia.org/wiki/Polarizer
http://www.olympusmicro.com/pr...olor/polarization.html

For a light wave to be absorbed by a molecule, it must be in a specific orientation relative to the molecule. For instance in infrared absorption spectroscopy, the light must be oriented parallel to the bond to be absorbed by the bond and add to its vibrational energy. I'm sure there are many different ways molecules can absorb light depending on the wavelength in question, but most will require orientation with something in the molecule. In fact infrared spectroscopy takes advantage of this in a technique known as dichroism to determine the alignment (or lack thereof) in molecules.

So with this piece of information, the first thing I would like to clear up is that the material in polarizers is never a "chaotic array of molecules". In science, this would be referred to as isotropic, meaning that the property in question is not dependent on angle, or the same in all directions. The fact that polaizers can polarize light directly evidences the fact that they are anisotropic, not the same in all directions. The olympus link above gives some ideas about how this is done. It can be as simple as stretching plastic in the melt state before cooling so that the polymer chains are aligned in one direction, or sometimes non-spherical nanofillers are added that also align during stretching.

As for chiral molecules, they will rotate the orientation of light, but can only polarize it if the bulk material somehow orients itself so that absorption is occuring preferentially in one direction. But, polarizers are very useful for viewing oriented molecules because of the fact that they rotate light. This is the underlying concept of polarized optical microscopy that allows me to view crystals, liquid crystals, and stress birefringence. Essentially, a polarizer polarizes incoming light before it meets the sample. Then, if the sample can rotate light (most organics) and can do so preferentially in one direction (generally requires and ordered structure) then after the light passes through the sample and meets another polarizer oriented at 90 degrees to the first, only light that has been rotated can pass through. This makes for very beautiful pictures of lighted crystals on a black background.

Hope this helps.

-Tim