Video explanations:
A pub-appropriate (silly) explanation of what polarimetry is by Jamie Lomax and myself:
A likely better explanation, with applications to molecule chirality from Steve Mould:
Want to get a little more in depth? Try this video from Eugene Khutoryansky:
Written explanations:
(This page is still a little rough. I’m editing it to make it more readable and making some visualizations.)
If you don’t recognize the word polarimetry or remember the details of how it works, you are not alone. Few astronomers today (outside of radio astronomy) regularly use the technique and many (including myself in graduate school) can’t remember the details of how it works when it’s brought up. Yet there is a wealth of information to be gained by considering the polarization of light, and it may be key to distinguishing between scenarios in some cases.
For these reasons, and for every curious mind, I’ve created a friendly explainer here. Answers to these common questions are broken into “Simply” (simple language) and “In more detail” explanations in case you are already familiar with some of these ideas. If you find a mistake please contact me.
Where is polarimetry most useful?
Simply:
Polarimetry is useful in cases where an observer wants to distinguish between scenarios such as a thick clear atmosphere, a surface, or a cloud deck, or determining the species of condensates in an atmosphere. This is because phenomena caused by these things such as ocean glint and rainbows are strong polarizers. In some cases it can also aid differentiating the light source: some stars produce very little polarized light, allowing us to see the planet beside them more clearly. It is also sensitive to where on the disk of the object the polarization is coming from, so in theory you could tell a world had a strongly polarizing polar haze without resolving it.
More Detail:
The quality of polarimetric observations depends—as most observations do—on one’s ability to collect photons; in polarimetry light usually is split to measure contributions in different orientations, so light collection needs to be especially strong. That being said, scientific polarimetry is done on relatively small telescopes. This is in part because “noise” contributions are reduced in some environments (for example most stars contribute very little polarization so polarimetry naturally nulls the star’s light).
The approach breaks degeneracies with other methods because it measures an additional component of light (you can still get intensity and wavelength information with polarimetry, polarimetry just adds the component of the electric field’s orientation). Because some processes polarize better than others, this approach can make it easier to pick them out or discern them from other sources. Ocean glint strongly polarizes light (read about its application here: Kopparla 2018 and citations therein). Rainbows are strongly polarized too, making cloud/condensate characterization one of the main applications for polarimetry. With high quality data one can distinguish the size, species and shape of condensate “droplets”. In some cases, one could also tell if they are encountering a cloud deck or not on a planet enabling better analysis of absorption lines (see Fauchez 2017).
To see some examples of where polarimetry has been historically key to interpretation, check out the History of Polarimetry page.
What is Polarized Light?
Simply:
Maybe you’ve heard this idea that light can be thought of like a wave. That wave can bounce off things and interact with things. The properties of the wave tell us stuff like how strong the light is or what color it is (how tightly/quickly (blue) or spread out/slowly (red) the waves come). Light is a type of wave that can wiggle from side-to-side, or up and down, or in a corkscrew, but “normally” wiggles in any and every direction with no preference.
This light without a preference is un-polarized light. Light that tends to wiggle its waves just up and down or just side-to-side is (linearly) polarized light. Because light that shows a preference for the direction it wiggles must have interacted with something (the surface of water, gas in our atmosphere, the molecules inside a crystal) to give it that preference. We can learn something about what environment the light was coming from by examining how it wiggles.
More detail:
If we examine light in terms of its electric field component, we find that in most cases that electric field has no preference in how it oscillates. That is to say, light from a star or incandescent light bulb has an electric field that oscillates in any and every direction. This is unpolarised light: the electric field has no preference for how it oscillates.
When light is reflected off a surface, or scattered through an atmosphere the electric field of the light that propagates in a given direction can have a preference in the direction it oscillates. The electric field may now oscillate side-to-side (linearly polarized light) or in a corkscrew (circularly polarized light).
In reality, the polarization is a quantum property of light. So it’s not that the wave of light oscillates in every direction, but rather that there is no measured preference to the spin of the collective photons. And linearly polarized light, it turns out, is just a superposition of photon spins with their oscillations in sync. The quantum nature of polarization is perhaps best exemplified by the popular example of a third polaroid placed at an angle between two others orthogonally aligned to each other.
HOW do these processes polarize light?
Light can be polarized through transmission, reflection, refraction, or scattering. The actual mechanism in each case owes to a preference in the direction of the photon correlated to the polarization oscillation. So bifringent media can pass on one orientation in a particular direction, and others in yet another direction. This can also happen with dichroic media (which also sort the light by wavelength, or color). Surfaces interfaces can split light by polarization similarly. In these cases the reflected and transmitted or absorbed orientations are allowed owing to the orientation of molecules. For water, for example, the reflected light is polarized because the molecules at the surface are consistently oriented in a particular way, and only allow certain oscillation directions to move through them.
In the natural world, polarization off of dirt, dust, ice, water, cloud droplets, and through media like atmospheres or crystals is often a combination of factors. One useful thing to keep in mind is that multiple scattering (the photon ping-ponging around in a diffuse medium like a powdery surface or dust, tends to depolarize light (there are some very interesting exceptions to this!) because it increases the chance that the polarization directions is randomized as the photons are reflected different numbers of times.
Polaroid materials polarize light through transmission by only allowing the light with, and E field oscillating in, one plane to pass through it. Unpolarized light passing through a polarizing filter, or polaroid will become polarized as the other directions of oscillation are not permitted. In a polarimeter this effect is used to detect the direction of polarization, that is, when the filter’s permissivity axis and the light’s polarization axis are aligned on ewould receive the strongest signal, when they are orthogonal no light passes through.
On a microscopic level the polaroid filter is comprised of molecules aligned in one direction throughout.
A similar thing occurs when a crystal is used: in some crystals there is one direction of propagation in transmission in which the electric field can oscillate in a given direction (polarization).
The real question becomes will those light waves cancel each other out because the timing is off. This can certainly happen—waves can be out of phase (you’ve perhaps seen this at the beach watching waves or ripples overlap and finding sometimes they build to bigger waves, sometimes they cancel each other out). Life seems to do some things that get around this signal loss. But it’s important here to consider that at worst this cancels the strength of the signal. It doesn’t change the direction the light seems to “twist”.
Finally looking at Part C, we can see that taking the mirror of that slinky, whatever direction it may be pointed (D), is what gives us light circularly polarized in the other (anti-clockwise) direction. It is only in this case, with the light twisted the other way and the two beams mixed in equal parts that would we cancel out our polarization.
More detail:
In the cartoon below in Part A we have circularly polarized light that to the observer seems to turn clockwise as the wave propagates towards them. Even if we change the direction of propogation of that wave it still appears to turn clockwise (B). This is to say it still twists the same way. It is like a “handedness” in three dimensions (one’s right hand flipped over does not make a left hand). When we consider the panels C and D we see light that to the observer the light propagates towards the electric field vector always appears to “twist” anti-clockwise with time.
Polarized light out of phase can certainly cancel out its own signal but in circularly polarized light, the direction of propagation varying would not cancel the signal unless the light was perfectly out of phase and coincident. Importantly this nulling, if imperfect, is only damping the intensity, not the direction or polarization itself. Nature doesn’t tend to produce light perfectly out of phase and coincident; in fact, in some cases molecules even amplify the circularly polarized light signal.
##A signal perfectly in phase with opposite chirality (clockwise to anticlockwise) would cancel the polarization but the signal you’d actually measure the Stokes parameters changing over time.
As you might imagine I get asked what polarimetry is, what Stokes parameters are, and other polarimetry related queries a lot.
To be fair, I had no idea (or could not recall?) how Stokes parameters worked when I started working in polarimetry. So, I thought I’d create a friendly explainer here. I’ve broken this into parts in case you are already familiar with some of these ideas. A simple language explanation precedes a more detailed one in each answer.
What are Stokes Parameters?
Simply:
To describe which direction the light wiggles—whether it’s more side-to-side or up-and-down or something in-between— we use Stokes parameters. There are four of them: I, Q, U, and V. I is the intensity and tells you how strong the light is, Q and U are for linearly polarized light. V is for circularly polarized light. Q measures how up-and-down vs side-to-side the light is. U measures the same thing but in slightly tilted coordinants—that’s really just to help us understand if the light isn’t perfectly polarized (it maybe tends to wiggle side to side but with a little up-and-down. V will tell us if corkscrew, twisted light winds clockwise or anti-clockwise.
More detail: There are a few ways to described the direction of the preference of the electric field oscillations in light. Stokes vectors are the most ubiquitous approach to this. Of the four Stokes vectors, I, Q ,U, and V; I is the intensity, Q and U measure linear polarization. Q is offset from U by 45 degrees. This allows for the description of imperfectly polarized light as both Q and U switch signs at 90 degrees, that is to say -Q is orthogonal to +Q. We can see that if we align our Stokes coordinate system just right we could have a polarization in only one Stokes parameter (Q and not U for example).
(To understand why we need U offset 45 degrees, consider a scenario where we use Q and U at 90 degrees (rather than pos/neg signs): here we might have cases where we have a value in Q of 1, and a value of U of 0.5. We know then that the light is not perfectly polarized (elliptical) but don’t know if that ellipsoid is tilted with respect to our coordinate system (with them offset 45 degrees with positive and negative signs we would see very strong values in one Stokes parameter and a weaker value in the other in this scenario).
Finally Stokes V describes circularly polarized light, providing its chirality (does it wind clockwise or anticlockwise with the propogation of the wave?). We can think of this as linearly polarized light that changes orientation in a consistent way with time. If you’re familiar with the use of half wave plates to measure polarized light, this might clarify why in most cases circularly polarized light is measured with a quarter-wave plate.
Why doesn’t circularly polarized light cancel itself out?
Simply: Think of a slinky or spring stretched like in the image below. This represents where the electric field (wiggle) points as the light wave moves forward (propogates). The direction the light wave is moving is important here. If light is always wound one way, with, say, the clockwise polarization shown in part A of the figure, we would see strong circular polarization. If the lightwaves move in the opposite direction (part B of the figure) the light isn’t moving towards us so we won’t see it, but importantly the polarization, the way the light winds as it moves forward is still the same. The polarization is still clockwise in regards to the direction of propagation (our “little green men” in the image would see it with the same polarization).
The real question becomes will those light waves cancel each other out because the timing is off. This can certainly happen—waves can be out of phase (you’ve perhaps seen this at the beach watching waves or ripples overlap and finding sometimes they build to bigger waves, sometimes they cancel each other out). Life seems to do some things that get around this signal loss. But it’s important here to consider that at worst this cancels the strength of the signal. It doesn’t change the direction the light seems to “twist”.
Finally looking at Part C, we can see that taking the mirror of that slinky, whatever direction it may be pointed (D), is what gives us light circularly polarized in the other (anti-clockwise) direction. It is only in this case, with the light twisted the other way and the two beams mixed in equal parts that would we cancel out our polarization.
More detail:
In the cartoon below in Part A we have circularly polarized light that to the observer seems to turn clockwise as the wave propagates towards them. Even if we change the direction of propagation of that wave it still appears to turn clockwise (B). This is to say it still twists the same way. It is like a “handedness” in three dimensions (one’s right hand flipped over does not make a left hand). When we consider the panels C and D we see light that to the observer the light propagates towards the electric field vector always appears to “twist” anti-clockwise with time.
Polarized light out of phase can certainly cancel out its own signal but in circularly polarized light, the direction of propagation varying would not cancel the signal unless the light was perfectly out of phase and coincident. Importantly this nulling, if imperfect, is only damping the intensity, not the direction or polarization itself. Nature doesn’t tend to produce light perfectly out of phase and coincident; in fact, in some cases molecules even amplify the circularly polarized light signal.
##A signal perfectly in phase with opposite chirality (clockwise to anticlockwise) would cancel the polarization but the signal you’d actually measure the Stokes parameters changing over time.
HOW do these processes polarize light?
Light can be polarized through transmission, reflection, refraction, or scattering.
Polaroid materials polarize light through transmission by only allowing the light with, and E field oscillating in, one plane to pass through it. Unpolarized light passing through a polarizing filter, or polaroid will become polarized as the other directions of oscillation are not permitted. In a polarimeter this effect is used to detect the direction of polarization, that is, when the filter’s permissivity axis and the light’s polarization axis are aligned on ewould receive the strongest signal, when they are orthogonal no light passes through.
On a microscopic level the polaroid filter is comprised of molecules aligned in one direction throughout.
A similar thing occurs when a crystal is used: in some crystals there is one direction of propagation in transmission in which the electric field can oscillate in a given direction (polarization).
On a microscopic level the filter is comprised of molecules or crystals (liquid crystals in the case of our polarimeter) aligned in one direction throughout. The light vibrating in that same direction is absorbed while the orthogonally aligned light is the component which passes. The alignment of the molecules effectively in unidirectional strings means that their degree of freedom is orthogonal to the "string". A polarising filter is not getting rid of the light that isn't aligned with the transmission axis, but rather is modifying the polarity of the light. In the case of the ferroelectric liquid crystal modulator used in our instrument, the voltage applied to the crystals unwinds them, changing their polarising orientation property (without a current applied they act as a half wave plate).
Light can be polarised by reflecting o a surface such as a liquid body, as we see in the glint o the ocean. Metallic surfaces do not polarise light as they reflect light with many directions of vibration. Non-metallic surfaces such as water will polarise light parallel to
the surface since the vibrations of the molecules will tend to align this way. This occurs because the electrons in the water or other material act like dipole radiators which will not transmit energy along their vibrational axis. The vibrational axis at the surface of water is aligned to the surface| so the transmitted light is perpendicularly polarised, but the reflected light is polarised along the parallel axis.
When light is refracted it becomes polarised to some degree. This is related to the polarisation produced by reflection, as the light that is reflected tends to be polarised parallel to the surface and the light refracted as it enters the material, such as water, is polarised perpendicular to the surface. In birefringent material, such as calcite, the light can be refracted at two different angles and thus with two different polarisations. In most cases light polarises with an axis normal to the surface; in calcite one beam will polarise normal to the surface and the other parallel.
Scattering can produce polarisation and is the primary process expected to be behind the polarised light observations discussed in this thesis. The repeated absorption and reemission of light can be anisotropic and anisopolar as in regular Mie scattering, or, depending on the particles scattering the light and the wavelengths undergoing the process, there can be isotropy and a tendency for a particular angle of polarisation as well. Rayleigh scattering is an example of this.
Mechanisms for polarisation in the material, which polarise light interacting with it, rely on the polarising material containing ions or dipoles. The exception to this is when electronic polarisation takes place (this is due to the charge asymmetry in the electron cloud).
The type of polarisation that occurs depends heavily on the type of material; those comprised of a single element will produce electronic polarisation, and some materials, such as water, will be capable of producing electronic, ionic, and dipolar polarisation as the molecules are mixed elements and dipolar. In an atmosphere a combination of these mechanisms can occur.
In addition, the wavelengths of light affected by these different mechanisms vary. Blue light, where we see Rayleigh scattering in most atmospheric conditions, is sensitive to electronic polarisation. Slightly longer wavelengths will begin to show sensitivity to ionic polarisation. Phenomena such as rainbows, Rayleigh scattering, effects from magnetic fields and glint produce signatures of linearly polarised light.
A related manifestation of polarised light that is not discussed here in detail but could one day be used to detect biosignatures in the form of chiral molecules is circularly polarised light. Circularly polarised light is a spiralling vector in polarised light, which is akin to two linearly polarised light vectors out of phase by a quarter of the wavelength and with an axis of vibration at ninety degrees from one another.