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At work, a senior colleague thinks that circularly polarized light does not exist. My problem is that we both work on a project involving polarized light. In some occasions, I would like to point out some artifacts that I can identify in the circularly polarized component (because I am computing the Stokes parameter that describes exactly that). My colleague insists to dismiss those arguments of mine on the basis that circularly polarized light does not exist. Instead, he argues, I should measure linear polarized states, and reason from there.

How can I show that circularly polarized light actually exists, and as a pure state? Or am I wrong?

My line of reasoning is that transversal waves are solutions of the wave equation. As this equation is linear, a linear combination of solutions is also a solution. And in that linear combination, I'm free to use complex coefficients. Hence, circularly polarized light is perfectly possible as an inherent property of light.

Now it turns out that my colleague is not particularly well versed in math, so my line of reasoning does not fly a single nanometer. Besides, it would be advantageous to me if I can also get someone else on my side, specially on the management floor, and math is certainly not the way.

I've read that there exist chiral molecules, which modify circularly polarized light, but do not affect linear polarized light. Some sugars, and also some oil-derivatives. But I could not find an explanation of the effect, much less an intuitive one.

Edit My colleague knows the effect of a QWP very well, but, he still thinks that circularly polarized light is a mental construction. More or less like some statistical blah-blah from the newspaper, that one is advised not to trust, unless one really knows what is going on. When I speak of Stokes parameters, he tends to mistrust the thing. I personally find math arguments very helpful to stay on safe ground, but it is also very difficult to communicate that feeling of safety to others. The guy is neither stupid nor obtuse, by any stretch of those concepts. In a way, I find it sane that he asks for arguments.

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  • $\begingroup$ Not sure what kind of stuff a guy like that will accept. Here is a well-known physics equipment supplier explaining how to make such light. And modern 3D movies use circularly polarised light because the old linearly polarised versions required viewers to keep their head fixed, which is a bad viewing experience. Here is a guy talking about this. In any case, it is a choice whether we want the pair of circular polarisations, vertical v.s. horizontal polarisations, or diagonal and anti-diagonal polarisations. $\endgroup$
    – naturallyInconsistent
    Commented May 13 at 4:48
  • $\begingroup$ In fact, if we want to consider light of fixed angular momentum, it is necessary to consider circular polarisation. It is this that corresponds well to spins, helicity and chirality. i.e. in a sense, it is circular polarisation that is fundamental, and the linearly polarisation is less so. However, of course, mathematically, any choice of basis is fine. Whatever is convenient. $\endgroup$
    – naturallyInconsistent
    Commented May 13 at 4:52
  • $\begingroup$ How does your colleague explain that circularly polarized light is partially transmitted though a linear polarizer, independent of its orientation? Does he claim the light is then unpolarized? If so, how would he explain the results of experiments that produce different outcomes for right-hand and left-hand circularly polarized light? Does he know what waveplates are, and if so, what does he think they do? A sugar + water solution acts as a waveplate and could be used to convert linearly polarized light to circularly polarized light and vice versa, I wonder how he would explain that. $\endgroup$
    – Puk
    Commented May 13 at 5:35
  • $\begingroup$ I suppose that if he doesn't like helicity he won't like the Poincaré sphere either. But as naturallyInconsistent said, "mathematically, any choice of basis is fine. Whatever is convenient". $\endgroup$
    – PM 2Ring
    Commented May 13 at 7:05
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    $\begingroup$ To play devil's advocate: technically you should be able to describe any optical effect either in terms of linear polarization or circular polarization, since both types form a basis for the polarization states of light propagating in a particular direction at a particular frequency. (Of course, it's often easier to do it using one basis rather than the other.) $\endgroup$
    – Michael Seifert
    Commented May 13 at 13:11

4 Answers 4

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With two circular and one linear polarisers you can do it. Use an unpolarised light source. First show that half the light passes the two circular polarisers when aligned and none passes when they are anti-aligned. So there is polarisation. Then replace the second filter by the linear filter and show its orientation does not matter. So the polarisation is not linear.

If that does not help, repeat the experiment by Richard Beth in 1935. Or get your colleague fired.

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  • $\begingroup$ The arguments making use of the angular moment might do the trick (my colleague is a mechanical engineer), specially if I find the way to connect to an experiment. I haven't thought of that at all. Now reading the 1936 article of Richard Beth. $\endgroup$
    – Y bueh...
    Commented May 13 at 6:46
  • $\begingroup$ @Ybueh... I gave you two videos in the question comment above. There are experiments in there. You can just ask him to take it up with actual vendors. $\endgroup$
    – naturallyInconsistent
    Commented May 13 at 8:21
  • $\begingroup$ @Ybueh... not worth arguing with you colleague ... it probably gives the same result no matter how you look at it. $\endgroup$
    – PhysicsDave
    Commented May 13 at 23:32
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In addition to my2cts answer showing how circularly polarized light is detected, I would like to add the reason for this phenomenon.

EM radiation can be polarized in the next way. In a polarization plate suitable for a certain frequency range, 50% of the radiation is directed in the polarization direction and 50% is absorbed/reflected. There is therefore an interaction with the electric field component of the radiation and the slits.

In waveguides it is more complex. It is assumed that both the magnetic and the electric field components can be aligned and that they can be aligned at an angle other than 90°.

In optical crystals, it is the periodic electric and magnetic fields of the crystal structure that polarize and diffract light. Among these, there are crystals that have two optical axes (birefringence, polarization planes, ...)

And there is the phenomenon that the radiation receives a twist when it emerges from an optical structure. We then observe this as circularly polarized light.

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Circular polarized light is photons in helicity eigenstates. I think the left vs. right is a bit confusing, but that is just convention. A photon in the spin state:

$$ \chi_{\pm} = |S=1, S_z = \frac{\pm 1} 2 \rangle $$

is left (right) or right (left) CP.

Linear polarization states are linear combinations of those.

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    $\begingroup$ Not an answer as the colleague us not likely to understand ‘helicity’. $\endgroup$
    – my2cts
    Commented May 13 at 5:04
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    $\begingroup$ @my2cts lucky I didn't go with "It's the $(\frac 1 2, \frac 1 2)$ irrep of the Lorentz Group. tl;dr Duh!?!?!" $\endgroup$
    – JEB
    Commented May 13 at 13:55
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Sorry I agree with your colleague. Circular pol light is typically a mix of 2 linear polarizations created by certain crystals/materials, the circular thing is a mathematical construct where we can add 2 vectors and get a phase .... phase can be interpreted in angular/radial dimensions.

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/quarwv.html

"Any linearly polarized light which strikes the plate will be divided into two components ....."

https://en.wikipedia.org/wiki/Birefringence Birefringence is necessary to produce the 2 component/circular light.

https://en.wikipedia.org/wiki/Circular_polarization Note how in Wikipedia the first 3 references are from the 1800s! Circular light is a long running convention in physics.

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    $\begingroup$ This is similar (not identical) to having non-polarized light, where actually the wave on average oscillates in every direction perpendicular to propagation vector. Just select a moment in time. $\endgroup$
    – Radek D
    Commented May 13 at 13:41
  • $\begingroup$ @RadekD yes but in the circular case we can use a simple polarizer to examine the light and get 2 maximums at 90 deg to each other. $\endgroup$
    – PhysicsDave
    Commented May 13 at 14:03
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    $\begingroup$ It is indeed correct to say that a circularly polarized wave is the vector sum of two linearly polarized waves, but it is equally true to say that a linearly polarized wave is the vector sum of two circularly polarized waves. One is no more correct than the other. $\endgroup$
    – hyportnex
    Commented May 13 at 14:13
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    $\begingroup$ @PhysicsDave For circular polarization, the transmitted power will be the same regardless of the orientation of the linear polarizer. There will not be maxima or minima of transmission, just as with unpolarized light. By your reasoning you could just as easily argue that linearly polarized light doesn't exist, due to hyportnex's point above. $\endgroup$
    – Puk
    Commented May 13 at 15:17
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    $\begingroup$ Note that the specific argument of OP's colleague is: "...some artifacts that I can identify in the circularly polarized component (because I am computing the Stokes parameter that describes exactly that). My colleague insists to dismiss those arguments of mine on the basis that circularly polarized light does not exist. Instead, he argues, I should measure linear polarized states, and reason from there." This answer does not touch on why it would be wrong or worse to make arguments based on the concept of circularly polarized light, just that one can technically do it without. $\endgroup$
    – JiK
    Commented May 13 at 15:30

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