Connections for the STEM Classroom

Polarization

Geoff Lenters, GVSU Physics Department

Universe exploration can only occur through our observation of the light signals sent from sources out there in the universe (at least at the present time – Solar System exploration can occur through the use of remote satellites; however, even these remote observers rely largely on light signals). We have observed this light at all different wavelengths from the radio region of the electromagnetic spectrum on up to the gamma ray region. Recently, archival searches have discovered previously unknown light sources in the radio region of the spectrum.

The light comes in very short bursts of radio waves. The bursts are only a few milliseconds in length and so far, no other emission in different parts of the electromagnetic spectrum have been observed in connection with these events. So, we don't really have any idea what the source of these bursts are, but we do have a clue from something known as polarization.

In order to understand polarization, we need to understand a little more about light. Light is modeled as something called an electromagnetic wave (there is a complementary model which understands light to be made up of bundles or particles of energy known as photons, but that's another story). Before we can understand an electromagnetic wave, we need to remind ourselves about ordinary waves such as water waves, or sound waves, or waves on a string. Generally, waves are observed by looking at the medium in which they travel. For instance, water waves travel through the medium of water. We watch the surface of the water to see the waves. The way in which the medium oscillates or "wiggles" relative to the direction of travel of a wave tells about the polarization of the wave. Watch a water wave as it propagates towards the beach sometime. The peak of the wave travels through the medium of the water towards the beach, but the medium itself -- the water -- oscillates up and down as the wave passes through it. This oscillation is transverse (perpendicular to) to the direction of propagation and so a water wave is known as a transverse wave (sound waves oscillate along the direction of propagation and are known as longitudinal waves). The direction of oscillation of the medium is labeled as the polarization of the wave. For water waves the oscillation has to be up and down because of the mechanism behind water waves. But, now, if you think about transverse waves on a string, you can imagine getting the string to wiggle in any of 360 degrees of direction. The direction of the "wiggle" is the polarization of the wave.

Polarization

Electromagnetic waves are transverse waves, but are different than water waves or waves on a string in that they don't require a medium to "wiggle" and the speed with which they propagate through space is a fundamental constant. We won't worry, here, about what is actually wiggling (if you're interested, it is electric and magnetic fields). But because there is something wiggling, and the wiggling is transverse, electromagnetic waves have polarization. It is this feature that makes polarized sunglasses work, for instance. Light reflected off of some surfaces gets polarized so that the wiggles are parallel to the surface, so for example, light reflecting off the water is polarized parallel to the water's surface and is said to be linearly polarized. Polarized sunglasses block light that is linearly polarized in a horizontal direction and so the reflected light from the water's surface is essentially blocked by the sunglasses. You can check sunglasses for polarization by looking through the glasses at a reflection off of a tile floor. Start by holding the sunglasses in the same orientation they would be if you were wearing them. While looking through the sunglasses, the reflection should be absent or muted if your glasses are polarized. If you then rotate the glasses, the reflection should come back or be brighter. There are many other ways in which polarization effects your everyday life. For instance 3D movies now use polarized light to generate the 3D effect (it began this way, then we went to a color scheme and now we are back to polarized light). The screens of your favorite electronic devices emit linearly polarized light and as a result, "security" screens can be used such that you can only see your screen if you are directly in front of it. You can check this out with your pair of polarized sunglasses!

In any case, the polarization of light can tell us something about the process that generated the light. Even more strangely, polarized light can be linearly polarized, elliptically polarized, or circularly polarized. If you want to understand circular polarization or its cousin, elliptical polarization, look at children playing "double-dutch" with jump ropes (do they do this anymore?). The ropes are in a state of circular polarization because the wiggles are rotating around the axis of the rope. The wiggles in electromagnetic waves can also do this and this is just what we found with the fast radio bursts at the beginning of all this. They are circularly polarized. What does this suggest? It suggests strong magnetic fields are at the source of this radiation. This narrows the down the list of astrophysical sources which could produce something like this -- fun things like black holes, neutrons stars, magnetars, and white dwarfs.

You might find the electromagnetic wave simulations (the first link) and acoustic (sound) wave simulations (the second link) at the following websites fun and interesting:

Electromagnetic Waves

Animations Illustrating Acoustics and Vibration by Dan Russell