[roesel]

While this explanation is very nice, it still does not actually explain what is happening on a material level.

The light does not "pass" through the middle filter, but it excites oscillations in the material, which effectively re-emits the light with different properties. The incoming light polarized at 0° induces oscillations in electrons which are "bound to a rail" in the material, which allows them to only oscillate in the direction of 45° (and all oscillations in the direction of -45° are absorbed). Therefore, a portion of the incoming field essentially gets re-emitted by the middle filter linearly polarized at 45°.

This representation is much less helpful if you think of the light in terms of individual photons rather than fields of course, but it is not worse than the article in this regard either.

[function_seven]

If the material is being excited into oscillations that then re-emit "new" light, how is the color and direction preserved? Polarization filters tend to pass the full spectrum (or nearly so) of visible light, but my understanding of photon absorption and emittance is that the wavelengths are dependent on the electron energy levels. (I'm thinking of the same mechanism that produces lines on a spectrometer, indicating which elements are present in a sample.)

I guarantee I've misused a term or two above. Hopefully you get what I'm asking.

Taking a stab at my own question, the "rails" are field lines within the material, and not electrons themselves that interact. Is that close?

[amluto]

It’s because the “re-emission” is coherent in the sense that it’s in the same phase as the incoming light. As a decent analogy: when you sing a pure note, it “excites” (vibrates) air molecules as it travels, and those air molecules in turn bump into other molecules, all at random, but still all in phase so that whoever is listening hears the original note. Similarly, when light goes through ordinary glass, it wiggles the electrons in the glass, which in turn change the way the light propagates, refracting it while still preserving an image.

Any textbook on electricity and magnetism will cover this in a section called something like “Maxwell’s equations in materials”.

[roesel]

I have used the word "re-emit" in the sense of the Maxwell equations description of electromagnetic fields. You are right that the light definitely does not get absorbed and then re-emitted in the sense you mean. In such a case, everything you wrote would be correct. (I probably should have used a term like "radiate" and perhaps dipoles instead of electrons to be more clear.)

I was trying to describe the propagation of light in a material, where the optical field induces oscillations in the dipoles of the material, and these dipoles in turn excite the optical field. This happens constantly in every nanometer of the material, and it is difficult to experimentally separate the field into the "material" portion and the "vacuum" portion, because it exists as an everchanging mixture as long as there are dipoles around.

As for the "rails", the way I've had it explained to me is that in one direction of a polarizer, electrons are free to move, so they fully absorb the light polarized in that direction. In the perpendicular direction, they are bound, and the best they can do in reaction to a field is oscillate back and forth a tiny bit. These oscillations excite an optical field again and it propagates further until it finds another dipole to excite. I like to crudely imagine a polarizer as the grid of a nanoscopic egg slicer :D. Field oscillations will get absorbed along the metal wires, but in the perpendicular direction, it will just excite vibrations in the wires, which will radiate them out again, sort of like a guitar string.

Let me know if this was helpful, or if I've made a mistake somewhere :).

[Rayhem]

It's very likely an off-resonant, non-quantum excitation. If the incident light is at the resonance frequency of the atom, it will excite electrons which will spontaneously decay and throw off the energy. Far from resonance, though, the atoms will wiggle (classically) like a harmonic oscillator from forces due to the fields (similar to the answer here: https://physics.stackexchange.com/a/474/23322).

[wyager]

This classical mechanics explanation works here because the process is linear. In a linear process, you can ignore photon quantization.

[moralestapia]

Is it photons in -> (new) photons out? Or the same ones reoriented?

[wyager]

It may not be possible to meaningfully answer this question in this case.

If we're talking about something like fluorescence, there's a fairy clear point where one photon disappears and another appears.

In a linear process like this, photons are not really absorbed by the material. In fact, the quantum behavior of photons is not relevant to the process, so you can just treat it purely as a wave phenomenon.

In cases like this, I would generally say that it is the "same" photon, but again, not really appropriate to think in terms of photons when there is nothing about the process that depends on quantization.

[roesel]

In my opinion, this question is impossible to properly answer in the framework of Maxwell's equations / field representation, since it cannot be defined in those terms.

If I were to answer this question in terms of photons as small amounts of field oscillations, I would argue that these are "new" photons, due to the fact that the "old" ones induced oscillations in the dipole moment of the material, which then in turn radiated energy out as the "new" photons.

But you can just as easily think of it as the material "suggesting" a better direction to the field propagating through it, and thus reorienting it. This is just very difficult to imagine and describe, at least for me.

[NotYourLawyer]

It’s new photons being emitted.

[amluto]

I disagree. Photons don’t have identity - you can’t distinguish old from new. This is true of all bosons, and it’s quite important to how they behave.

[mensetmanusman]

It’s both and neither since photons are particles and waves and focusing on one or the other to build intuition can be useful in some cases and not other.

How light actually behaves is probably beyond the ability of human cognition (since so much happens in a billionth of a second)

[wyager]

> How light actually behaves is probably beyond the ability of human cognition (since so much happens in a billionth of a second)

This is not remotely true. The behavior of light is very well understood and relatively simple to model compared to other, less linear physical processes.

[Retric]

We have some great models but what actually happens doesn't quite fit into any one model. For example all photons are constantly redshifted as they travel through space because space is expanding. That’s not really relevant on human timescales but it is an effect that takes place between your monitor and your eyes.

When you really dig into this stuff your realise stuff like the density of air is really an abstraction that doesn't quite fit what is actually going on.

[akomtu]

Can you think of a photon as a localized magnetic wave, a tiny soliton, stable due to properties of the magnetic field?

[moralestapia]

(Interesting) Could you elaborate?