By Briley Lewis
A brief introduction to polarized light
Light is an electromagnetic wave — and its electric field doesn’t always point in the same direction. The orientation of the light’s electric field defines its “state of polarization”. In this guide we will talk about what polarization is, how it is produced by the cosmos and how we can observe it.
We categorize polarization in three main ways: unpolarized light, linearly polarized light, and elliptically polarized light. Unpolarized light (i.e. natural light) is best described as randomly polarized light; that is, many light sources are a collection of emitters where the polarization of the emitted light changes very frequently and randomly. This is an extreme, and often the light is partially somehow polarized. Linearly polarized light has a constant electric field orientation (although the amplitude of the wave can still vary.) Elliptically polarized light has an electric field whose vector rotates, tracing an ellipse. One case is circularly polarized light, where the x and y directions have the same amplitude. Some of these cases are illustrated in the figure below.
We can mathematically describe polarization using matrices. Stokes vectors (also called Stokes parameters) are a useful way to do this. There are four parameters: I, Q, youand V. I is the total intensity, Q describes the linear polarization (horizontal or vertical, depending on the sign), and you describes the polarization at a 2nd set of orthogonal axes (+/-45 degrees), and V describes the elliptical polarization (right-handed if >0, left-handed<0). They are defined as follows:
- I = S1 = 2I0 (Or I0 is the incident light)
- Q=S2 = 2I1 – 2I0 (Or I1 is the light through a linear polarizer with a horizontal axis)
- U=S3 = 2I2 – 2I0 (Or I2 is the light through a linear polarizer with an axis at 45oh)
- V=S4 = 2I3 – 2I0 (Or I3 is light through a circular polarizer)
For fully polarized light, I2 =Q2 + U2 +V2. For a partially polarized system, the degree of polarization is given by P = (Q2 + U2 +V2)½ / I. See Hecht’s Table 8.5 for an illustrative example of Stokes vectors for various polarization states. Similarly, the operations of different polarizers on Stokes vectors can be described by Mueller matrices.
What in the universe creates polarized light?
Polarization can be affected by dichroism, reflection, scattering, or birefringence (more on dichroism and birefringence in the next section!), as well as other electromagnetic effects. Some radiation processes, such as synchrotron radiation, also naturally produce polarized light.
Light can be polarized by scattering due to interactions with electrons. For unpolarized incident light, light scattered along the incident axis will not be modified and light scattered at orthogonal angles (90 degrees) will be linearly polarized. Scattering can be more complicated depending on the size of the particle relative to the wavelength of the light: Rayleigh scattering describes what happens when particles are much smaller than the wavelength, and Mie diffusion describes diffusion more generally.
Light can also be polarized by reflection on a dielectric medium, where only one component of the incoming polarization will be reflected and the other will be refracted. Brewster’s Law describes the angle where the reflected ray will be fully polarized and deviations from this angle will be partially polarized.
Here are some examples of situations that create polarized light in astronomy:
How is polarization measured?
To determine how much incoming light is polarized, we need to use some kind of polarizer – a filter that separates light into its component parts or allows only a certain polarization of light to pass. As Hecht says in his Optical manual, for polarizers to work “there has to be some sort of asymmetry associated with the process”.
Some polarizers use dichroism, where only one polarization state is selectively absorbed, and the other orthogonal polarization state passes just fine. Some crystals are naturally dichroic, as are Polaroid filters. Another commonly exploited effect is birefringence, which means that a substance has different indices of refraction due to the arrangement of atoms within it. Some birefringent crystals can split light into orthogonal polarization states. A useful example in astronomy is the Wollaston prism, which serves as a polarizing beam splitter in many instruments.
Another important type of optics is known as a waveplate, something that changes the polarization of light in your incoming beam. A full wave plate creates a phase difference of 360 degrees (2π radians), whereas a half wave plate induces a phase difference of 180 degrees (π radians) and a quarter wave plate shifts the phase by 90 degrees (π/2 radians). There are also polarizers that induce circular polarization, such as the combination of a linear polarizer and a wave plate.
So what makes an astronomical polarimeter? At least in optics/infrared, there’s usually some kind of beam splitter, like a Wollaston prism, that splits the light into two orthogonal polarizations, plus a half-wave plate that allows the observer to modulate the polarization so to calibrate the instrumental effects. (You can read in detail about the Gemini Planet Imager Polarimeter here as an example!)
Beyond optical and infrared, there are also other ways to measure polarimetry. Radio telescopes can detect polarization because they essentially record the state of the electric field, and other types of detectors for high energy light like X-rays (e.g. gas pixel detectors) have also been designed to measure polarization.
Some current observatories with polarimetry capabilities and their interesting scientific results (as well as relevant Astrobites!)
IXPE [The Imaging X-Ray Polarimetry Explorer] — The IXPE mission recently launched by NASA will search for the polarization of certain extreme sources, such as supernovae, AGN and pulsars! Be on the lookout for its first results to come very soon.
VLT/SPHERE — SPHERE focuses on the characterization and detection of exoplanets, including the extremely cool detection of PDS 70b, a very young planet in formation still embedded in its disk.
Gemini Planet Imager — Briefly mentioned earlier, the Gemini Planet Imager didn’t just image planets, it also imaged debris disks! And he did it in polarized light, using differential polarimetric imaging, a technique that separates starlight from disc light. They have a full sample of Polarized Debris Discs, as well as extensive and careful studies of individual discs!
Subaru/SCExAO/CHARIS — The CHARIS instrument of the Subaru telescope can do spectropolarimetry [looking at polarization in multiple wavelengths] in the infrared, including polarimetric differential imaging (CHARIS-PDI) which is useful for finding exoplanets and disks. They made cool images of jets of young T Tauri stars and debris disks!
ALMA — Polarimetry works a little differently for radio telescope arrays like ALMA, but they get it right. ALMA has played a key role in understanding the magnetic fields of objects across the Universe, such as the interesting and extreme supernova AT2018cow!
Event Horizon Telescope — Similar to ALMA in that it’s a bit different from a “normal” single telescope, the EHT array has successfully measured one of the most extreme examples of polarization to date: polarized light from the region. dust around the supermassive black hole of M87!
HARPS — HARPS, ESO’s famous spectrograph, now has polarimetric capabilities! It is capable of spectropolarimetry, which can help understand the magnetic fields of stars.
SOFIA HAWC+ — The SOFIA Airborne Observatory has a unique far-infrared imaging polarimeter called HAWC+ that has been used to observe star forming regions and emission in a dusty torus around an active galactic nucleus.
There are certainly more polarimeters and science cases than mentioned here, but hopefully this is a useful start if you’re thinking about polarimetry in your research or just trying to find out more!
Astrobite edited by: Jessie Thwaites and Sabina Sagynbayeva
Featured image credit: Encyclopedia Britannica
Resources:
ESO Polarimetry
Polarimetry: a powerful diagnostic tool in astronomy
Astronomical Polarimetry (thesis)
[Book] Kolokolova, L., Hough, J., & Levasseur-Regourd, A. (Eds.). (2015). Polarimetry of stars and planetary systems. Cambridge: Cambridge University Press. doi:10.1017/CBO9781107358249
[Textbook] Hecht, Eugene. Optical. Pearson Education, 2012.
About Briley Lewis
Briley Lewis is a PhD candidate and NSF Fellow at the University of California, Los Angeles, studying astronomy and astrophysics. His research interests are primarily in planetary systems – both exoplanets and objects in our own solar system, how they form, and how we can create instruments to learn more about them. She previously continued her research at the American Museum of Natural History in New York, as well as the Space Telescope Science Institute in Baltimore, MD. Outside of research, she is passionate about teaching and public outreach, and spends her free time combining her love of science with her love of crafts and writing, and playing with her dog. Rescue Rocky.
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