Photon polarization and 1/2 wave plates

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Discussion Overview

The discussion revolves around the behavior of photon polarization when interacting with half-wave plates, specifically addressing the effects of orientation and the resulting polarization states. Participants explore theoretical implications, mathematical reasoning, and the nature of wave plates in altering polarization.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that a vertically polarized photon hitting a half-wave plate set at 90 degrees will not pass through, based on the cosine squared relationship.
  • Others propose that 50% of vertically polarized photons passing through a half-wave plate set at 45 degrees is due to the same cosine squared reasoning.
  • A claim is made that 25% of photons from the previous step will pass through a half-wave plate set at 90 degrees, following the same mathematical logic.
  • One participant questions whether the photons that pass through the second plate are polarized at 45 degrees and whether those passing through the third plate are horizontally polarized with unchanged energy.
  • Another participant suggests substituting "polarizing filter" for "half-wave plate" to clarify the distinction, asserting that a wave plate shifts polarization rather than blocking it.
  • Concerns are raised about the nature of wave plates, with some participants arguing that they only change the phase of light rather than its polarization state.
  • Further discussion includes the idea that while wave plates may not change the polarization of individual photons, they can affect the overall polarization state in a macroscopic sense.
  • One participant emphasizes that the frequency (or wavelength) of light is generally not affected by the wave plate, while another clarifies that they meant to refer to phase changes.

Areas of Agreement / Disagreement

Participants express differing views on the role of half-wave plates versus polarizing filters, the nature of polarization changes, and the implications of phase shifts. The discussion remains unresolved with multiple competing perspectives on the effects of wave plates.

Contextual Notes

There are limitations regarding the definitions of polarization and the assumptions about the behavior of light through optical devices. The discussion does not resolve the mathematical steps or the implications of the phase changes versus polarization changes.

zincshow
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I have seen this explained but would appreciate comments as to whether I understand it correctly.

#1. A vertically polarized photon hits a 1/2 wave plate set at 90 degrees will not pass through since cos(90)^2 = 0

#2. 50% of vertically polarized photons that hit a 1/2 wave plate set at 45 degrees will pass through since cos(45)^2 = 0.5

#3. following #2, 25% of the photons from #2 will get through a 1/2 wave plate set at 90 degrees since cos(90-45)^2 = 0.5 and 0.5*0.5 = 0.25

The question: does this mean that the photons that get through #2 are polarized at 45 degrees and that the photons that get through #3 are now horizontally polarized with the same energy that they started with?
 
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zincshow said:
I have seen this explained but would appreciate comments as to whether I understand it correctly.

#1. A vertically polarized photon hits a 1/2 wave plate set at 90 degrees will not pass through since cos(90)^2 = 0

#2. 50% of vertically polarized photons that hit a 1/2 wave plate set at 45 degrees will pass through since cos(45)^2 = 0.5

#3. following #2, 25% of the photons from #2 will get through a 1/2 wave plate set at 90 degrees since cos(90-45)^2 = 0.5 and 0.5*0.5 = 0.25

The question: does this mean that the photons that get through #2 are polarized at 45 degrees and that the photons that get through #3 are now horizontally polarized with the same energy that they started with?

First, you should substitute "polarizing filter" for "half wave plate" in the above for it to be correct. A wave plate passes everything, and shifts the polarization by a fixed amount.

Second, and with that in mind, the answer is YES: They are now horizontally polarized.
 
DrChinese said:
First, you should substitute "polarizing filter" for "half wave plate" in the above for it to be correct. A wave plate passes everything, and shifts the polarization by a fixed amount.

Second, and with that in mind, the answer is YES: They are now horizontally polarized.

I thought wave plates only change the phase, not the polarization.
 
LostConjugate said:
I thought wave plates only change the phase, not the polarization.

It has the same effect in a lot of cases, keeping in mind that the entangled photons do not have a well defined polarization to change. A half wave plate should change an H> to a V>.

From Wikipedia:

"A wave plate or retarder is an optical device that alters the polarization state of a light wave traveling through it. A wave plate works by shifting the phase between two perpendicular polarization components of the light wave... "
 
DrChinese said:
It has the same effect in a lot of cases, keeping in mind that the entangled photons do not have a well defined polarization to change. A half wave plate should change an H> to a V>.

From Wikipedia:

"A wave plate or retarder is an optical device that alters the polarization state of a light wave traveling through it. A wave plate works by shifting the phase between two perpendicular polarization components of the light wave... "

So it looks like according to the wiki article it does only change the phase, but this results in an interference that changes the expectation value of polarity to be perpendicular to the source.

In other words it does not change the polarity of any single photon, only the frequency. The combined effect of many measurements or a macroscopic (classical) measurement being shifted in polarization.

Do I have that right?

"A half-wave plate. Linearly polarized light entering a wave plate can be resolved into two waves, parallel (shown as green) and perpendicular (blue) to the optical axis of the wave plate. In the plate, the parallel wave propagates slightly slower than the perpendicular one. At the far side of the plate, the parallel wave is exactly half of a wavelength delayed relative to the perpendicular wave, and the resulting combination (red) is orthogonally polarized compared to its entrance state."
 
LostConjugate said:
In other words it does not change the polarity of any single photon, only the frequency.

Generally, I don't think the frequency (wavelength) is affected by such plate.
 
DrChinese said:
Generally, I don't think the frequency (wavelength) is affected by such plate.

Oh the phase I mean.
 

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