Polarizers kicking my behind!

sophiecentaur

Did you not notice the bump on my head and the plaster cast on my leg?

G01

That is not what is meant by active and passive. As sophiecentaur said, being 'active' implies a power source. http://en.wikipedia.org/wiki/Passivi...engineering%29

2112rush2112

I'm tellin' you bro, I have a PF Science Adviser (you) and a PF Gold Member and Homework Helper (G01) trying to explain this to me but it's still not sinking into my thick head! The polarizer is still kicking my ***!
I understand this idea--the idea that a polarizer allows more light to pass than only and strictly those photons that pass in the polarizer's plane.
Dr. David Goodstein of Mechanical Universe fame (remember that show?) tried to explain it quantum mechanically in an episode, but in doing so demonstrated that even Cal Tech faculty can be fallible. And here's where I run into difficulty. I'm trying to figure out how a polarizer can alter the polarization of light without being a half-wave plate. I thought only half-wave wave plates can do this (and even then, half-wave plates rotate light 90 degrees; not 45 degrees).
So you do agree with G01 that polarizers do in fact alter photons in some way, correct? Not classicly, like a racemic mixture or glucose would, but an alteration of the photon's plane of polarization on a more fundamental level.

I would like to learn more about what happens to a photon as it exits a linear polarizer (Polaroid, Nicol prism, etc.). My first instinct is to learn about E and H fields, but that's all property of James Clerk Maxwell, and over 150 years old. Thus, to gain a better understanding of nature, it's best to look beyond Maxwell (as much as I do revere him), no?

2112rush2112

Sh!t one hell of an orgasm.............

G01

Your first instinct is correct. The model that provides the better understanding is the one best suited to the problem. I agree with what sophiecentaur said earlier:

Emphasis mine. The problem can be understood in terms of photons, of course, but if you do not understand classical E and H fields yet, then focusing on photons is just going to make the physics of polarizers even more opaque. As all the physics of the problems is validly modeled with classical theory, you are not missing any of the physics by focusing on a classical picture for the time being.

sophiecentaur

My issue with trying to explain things in terms of photons is that they are not any more 'real' than fields. I can appreciate that it may be easier to have a picture of a little bullet travelling from A to B than some sort of nebulous set of invisible vectors out there (everywhere, in fact). The only thing we can measure, though, whatever we believe is happening between A and B, is the result, when we actually detect the energy. There are cases where the photon explanation fall out nicer and there are cases where the field explanation does it easier. There are also, say the two slits, where either model will give you an answer without too much trouble.

When you talk about getting a better understanding than Maxwell can give you, you need to be careful not to replace Maxwell with some more trivial model. The sort of photons that people use in these arm waving explanations are way more trivial than Maxwell. Any worthwhile photon model must include all the factors that Maxwell entails and then some. You can't replace Maxwell with something that's not as good and call that 'better understanding'.

The simplest form of polariser to consider is probably a dipole (or any straight wire) in an RF field. The classical way of treating it is to talk of the currents induced along the wire due, only to the component of the wave polarised parallel to the wire. and the re-radiation / reflection of energy in the form of waves that are polarised parallel to the dipole wire. The component of the incident wave normal to the wire is unaffected by it. All other polarisers do more or less the same thing; if it's happening in a solid then it's more complex but you could deal with that later. The polariser - because it is an object that is interacting with the wave - is a mechanism that 'measures' the states of the photons and resolves the uncertainty. When the photons have passed out of the other side, they are quantum objects again and could be anywhere, doing anything until they are measured again.

Applying photons to this. A photon has to have a certain energy so you can't have a 'component of a photon'. When the RF energy passes the wire, there is a probability that a certain proportion of the photons will interact with the wire. Surprise surprise, that probability just happens to correspond to half of the photons, when the incident wave is unpolarised (a random selection of radio signals from a massive selection of transmitters in all different orientations - that's like your ordinary light source). Moreover, if you are dealing with just one source (a linearly polarised signal), the probability of a photon interacting with the wire just happens to be proportional to the Cosine of the angle of orientation of the transmitting and receiving wires (the square of the E component). The good-ol' wave explanation predicts what will happen to the photons that we measure with our radio receiver. We can choose to believe that it was photons that jumped between the transmitter and receiver or we can say it was waves but, without putting another detector in the way (and messing up the experiment) we can't say which way it really happened. Those photons really have to have an instruction book with them to tell them how to behave like Mr Maxwell told them to and they can't ignore him.