Energy, photons, angular momentum and polarization

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Discussion Overview

The discussion revolves around the interaction of linearly polarized photons with quarter waveplates, focusing on the transfer of angular momentum, energy loss, and the effects of rotation on the polarization state of the photons. Participants explore both theoretical implications and experimental considerations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants propose that when a linearly polarized photon passes through a rotating quarter waveplate aligned at 45 degrees, energy is transferred from the photon to the plate, resulting in a loss of energy for the photon.
  • Others argue that if the waveplate is fixed, while torque is applied to the plate, there may not be any energy transfer, as the torque does not do work, likening it to a ball bouncing off a fixed wall where momentum is transferred but energy remains unchanged.
  • A participant questions whether the frequency of a photon changes when passing through a rotating quarter waveplate, suggesting that such a change would indicate energy loss or gain.
  • Some participants highlight that the moment of inertia of the plate is significantly larger than that of the photon, implying that the energy transferred to the plate is negligible.
  • One participant mentions that a realistic experiment would involve a pulse of light with many photons, prompting interest in quantifying the energy and angular momentum transfer in such scenarios.
  • Another participant references historical experiments involving angular momentum transfer with waveplates and expresses frustration over access to academic papers, advocating for open access to scientific knowledge.

Areas of Agreement / Disagreement

Participants express differing views on whether energy is lost by the photon in both scenarios, with some asserting that energy transfer occurs while others maintain that it does not. The discussion remains unresolved, with multiple competing perspectives on the implications of angular momentum transfer and energy dynamics.

Contextual Notes

Participants note that the assumptions regarding the moment of inertia and the nature of torque may influence the conclusions drawn about energy transfer. The discussion also touches on the limitations of theoretical models versus practical experimental setups.

alfredblase
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A linearly polarized photon passes through a quarter waveplate that is free to rotate and is aligned at 45 degrees to the polarizaton direction,,,
The wave plate will acquire an angular momentum,,, [Walborn, Cunha, Padua and Monken 2002, Physical review A, 65, 033818-1]

This means that energy is transferred from the photon to the plate, and that therefore the photon loses energy,, Am I correct in saying this?

Next question:

Now the same thing experiment is done but the wave plate is not free to rotate,, the photon will undergo exactly the same change as before, [i,e it will change from linearly polarized to circularly polarized],,

Would I be right in saying that the photon again loses energy by applying a torque to the plate? If not please explain in detail the difference between the first scenario and the second [refering to the physics],,

Help would be much appreciated
 
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alfredblase said:
A linearly polarized photon passes through a quarter waveplate that is free to rotate and is aligned at 45 degrees to the polarizaton direction,,,
The wave plate will acquire an angular momentum,,, [Walborn, Cunha, Padua and Monken 2002, Physical review A, 65, 033818-1]

This means that energy is transferred from the photon to the plate, and that therefore the photon loses energy,, Am I correct in saying this?

Well, if it starts rotating, you will of course not keep your alignment with 45 degrees, so you will end up having -45 degrees, and sending out the OPPOSITE circular polarisation ; the transfer is now in the other direction...

I don't know if you change the frequency of a photon by sending it through a *rotating* quarter-wavelength plate ; that's very well possible, and when the frequency of the photon changes, it has lost (or gained) energy.

Now the same thing experiment is done but the wave plate is not free to rotate,, the photon will undergo exactly the same change as before, [i,e it will change from linearly polarized to circularly polarized],,

Would I be right in saying that the photon again loses energy by applying a torque to the plate? If not please explain in detail the difference between the first scenario and the second [refering to the physics],,

Help would be much appreciated

It will apply torque to the plate, but as this torque does not do any work, I don't think that there is any energy transfer (and hence a frequency change of the photon).
This is a bit as a ball bouncing on a fixed wall: there's momentum transfer, but no energy loss.
 
thanks vanesch,

I don't know if you change the frequency of a photon by sending it through a *rotating* quarter-wavelength plate ; that's very well possible, and when the frequency of the photon changes, it has lost (or gained) energy.

hmm, I am afraid i need a definite answer :shy: Also I presume the plate was not rotating before the photon passed through it

It will apply torque to the plate, but as this torque does not do any work, I don't think that there is any energy transfer (and hence a frequency change of the photon).
This is a bit as a ball bouncing on a fixed wall: there's momentum transfer, but no energy loss.

I disagree, I mean, the things fixing the plate will be deformed very slightly no? like when i push my finger against a brick wall the wall deforms very slightly,,,
 
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alfredblase said:
A linearly polarized photon passes through a quarter waveplate that is free to rotate and is aligned at 45 degrees to the polarizaton direction,,,
The wave plate will acquire an angular momentum,,, [Walborn, Cunha, Padua and Monken 2002, Physical review A, 65, 033818-1]

1. This means that energy is transferred from the photon to the plate, and that therefore the photon loses energy,, Am I correct in saying this?

Next question:

Now the same thing experiment is done but the wave plate is not free to rotate,, the photon will undergo exactly the same change as before, [i,e it will change from linearly polarized to circularly polarized],,

2. Would I be right in saying that the photon again loses energy by applying a torque to the plate? If not please explain in detail the difference between the first scenario and the second [refering to the physics],,

Help would be much appreciated
1. The moment of inertia of the plate is so large compared with that of the photon that the energy given to the plate is completely negligible.
It is the same thing as when you bounce a ball off the floor. The Earth gains momentum, but negligible energy.
2. There is no difference. In the case of the "fixed" plate, it is the Earth that gets the angular momentum but only negligible energy.
In these cases, "negligible" really means negligible.
 
Of course, a physically realizable experiment would surely involve a pulse of light containing many many photons, not just a single photon.

It would be interesting to see some numbers for such an experiment: energy of the light pulse, amount of angular momentum transferred to the quarter-wave plate, moment of inertia of the plate, and the amount of rotational kinetic energy gained by the plate.
 
1. The moment of inertia of the plate is so large compared with that of the photon that the energy given to the plate is completely negligible.
It is the same thing as when you bounce a ball off the floor. The Earth gains momentum, but negligible energy.
2. There is no difference. In the case of the "fixed" plate, it is the Earth that gets the angular momentum but only negligible energy.
In these cases, "negligible" really means negligible.

sorry that argument doesn't really wash
 
I found the link: http://www.spie.org/web/oer/june/jun97/photon.html

Ok this website tells us that transfer of angular momentum has been carried out with 1/2 wave plates [Beth 1936], and with molecule clusters,, [Santamato and Shen 1986]

I find it ridiculous that the general public is not allowed access to these papers without paying tho,, is there some way I can read papers from subscription journals for free? I think knowledge should be available to all not just to those with a flush credit card,,

Anyway so apparently wave plates and molecule clusters are rotated thus gaining kinetic energy and so redshifting the photons

Views on this and confirmation/debunking of what I have said would be much appreciated,,

thanks
 
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