Adding a 3rd Polarizer Changes Light Passing Through It

[thenewmans]

One time, I saw this experiment where 2 polarizers were offset by 90 degrees so that no light got through. Then a third polarizer was added in between so that some light got through. I don’t know how this works in either a classical physics explanation or in a QM explanation. Do the polarizers adjust the spin or angle of the photon? If so, I wouldn’t have expected that. I thought that the photons that make it through a polarizer come out in just the same condition as when they entered.

I ask because I’m thinking about what implications this might have in an entanglement experiment.

 

[tiny-tim]

hi thenewmans!

… I thought that the photons that make it through a polarizer come out in just the same condition as when they entered.

nope, any thing getting through a "horizontal" polarizer comes out "horizontally" polarized

 

[JesseM]

Page on the "Dirac three-polarizers experiment" here:

http://www.informationphilosopher.com/solutions/experiments/dirac_3-polarizers/

Here's a java applet that allows you to recreate it:

http://www.colorado.edu/physics/PhysicsInitiative/Physics2000/applets/lens.html

One way of looking at this would be that traveling through a polarizer at a given angle "collapses" the photon's wave function so it is definitely able to make it through at that angle (and definitely blocked by a polarizer at 90 degrees to that angle), while at other angles it has some probability of getting through and some probability of getting blocked. You can probably also model this situation without the assumption of "collapse", just a wavefunction continuously evolving over time or doing a path integral to calculate the probability the photon will be seen at a detector past the final filter, though I'm not sure of the details.

 

[unusualname]

http://www.informationphilosopher.com/solutions/experiments/dirac_3-polarizers/

the third polariser is placed at an angle between the two orthogonal polarisers, now you have a random fraction which get through this third polariser and also another random fraction that get through the final polariser.

edit: oops, JesseM posted the same link :-)

 

[SpectraCat]

It is worth noting that there is a very deep similarity between the 3 polarizer example and the http://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment" . I personally consider this to be the clearest evidence of two fundamental properties of quantum mechanics:

1) quantum particles have complex phases associated with their positions/trajectories
2) the Heisenberg Uncertainty Principle .. i.e. the fundamental restriction on knowing precise values for non-commuting observables.

 

[San K]

It is worth noting that there is a very deep similarity between the 3 polarizer example and the http://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment" . I personally consider this to be the clearest evidence of two fundamental properties of quantum mechanics:

1) quantum particles have complex phases associated with their positions/trajectories
2) the Heisenberg Uncertainty Principle .. i.e. the fundamental restriction on knowing precise values for non-commuting observables.

How is the Heisenberg Uncertainty Principle for quantum mechanics validated by the results of the 3 polarizers?

 

[SpectraCat]

How is the Heisenberg Uncertainty Principle for quantum mechanics validated by the results of the 3 polarizers?

I was referring to the triple Stern-Gerlach experiment ... but I expect you can draw the same conclusions from the polarization experiment, if you include field quantization in your description. I am not an expert on QFT though, so I am not completely sure.

 

[SpectraCat]

Maybe you are thinking that the randomness that allows the photons to pass through all the three polarizers, can mathematically be used to prove inherent randomness?

No, nothing like that. I am thinking that the polarization direction of a photon is much like the projection of a spin-1/2 particle on a space fixed axis. The triple SG shows that you can only know one spatial component (x OR y OR z) of the spin vector. I expect that QED shows that there is an equivalent restriction on what you can know about the polarization direction of a photon .. but like I said, I don't know that to be true.

 

[thenewmans]

Wow, OK, all I got to say is this is why I love PF.