# Photon Spin and Polarization filters

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• PavanKumar
PavanKumar
I understand how polarization can be explained using EM waves. However, I am unable to understand how to explain how polarization filters work when we use the concept of photon spins. Can someone help me with that?

Gold Member
There's a really good video over at 3Blue1Brown that explains the quantum mechanics of photons passing through polarization filters:

They don't mention it, but when light is circularly polarized, each photon has a spin angular momentum of ##+\hbar## or ##-\hbar##, depending on whether the light is left or right circularly polarized.

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Gold Member
2022 Award
Be careful! A photon has no spin in the usual sense. Massless quanta have to be treated separately from massive ones. That's why in standard QFT massless particles have only 0 (for scalar and pseudo scalar fields) or 2 (for fields with spin ##\geq 1/2##) spin-like degrees of freedom (which most intuitively can be chosen in terms of the single-free-particle momentum-helicity basis ##|\vec{p},h \rangle## with ##h=\pm s## and ##\vec{p}## with the dispersion relation ##p \cdot p=0##, i.e., ##E=|\vec{p}|##).

For a first qualitative explanation of polaroids (absorptive polarization filters), see

https://en.wikipedia.org/wiki/Polarizer

Geofleur
Gold Member
Does that mean, instead of saying that the photon has spin 1, we should instead say that it has helicity 1? I didn't realize that the representations of the Poincare group for massless particles cannot be labeled by spin!

Staff Emeritus
Gold Member
Does that mean, instead of saying that the photon has spin 1, we should instead say that it has helicity 1? I didn't realize that the representations of the Poincare group for massless particles cannot be labeled by spin!

This is because the little group of a massive particle, SU(2), is different than the little group of a photon, E(2).

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