If we think about the wave-like properties of light, we might start to wonder exactly what direction the waves are vibrating in. Our best analogy for thinking about waves is water, and in that situation the waves vibrate in the direction perpendicular to the wave’s motion. That picture is somewhat limiting because there are really only two dimensions of motion, but it does give us the right concept. Light waves also vibrate perpendicular to its motion, but now it’s a lot more complicated because there are many directions that this could be.

The vibrations of light are typically oriented randomly. That is, each photon that arrives is vibrating in its own random direction, so that when you have lots of photons there is no coherence to the vibrations you are seeing. This is known as unpolarized light. Most of the light you encounter is unpolarized. It takes special conditions to get the light waves to become oriented in some type of meaningful pattern to become polarized light The most common types of polarized light in applications are linearly and circularly polarized light.

Linearly polarized light vibrates back and forth in a flat line pattern. This can be visualized as light that needs to pass through a series of parallel bars. Only the light that is aligned with the bars can make it through, and the rest of it gets blocked. And this is basically what a polarizing filters do. Our eyes cannot detect polarlized light directly, but we can test whether light is polarized by rotating a polarizing filter. If the light is polarized, the brightness will change depending on the angle. In fact, if all of the light is polarized, you will block all of the light if you hold the filter perpendicular to the polarization.

Polarized light can be formed off the reflection of surfaces, but only at certain angles (and the angle depends on the surface). In these situations, polarized filters can be used to block just the light off of that surface without blocking the rest of the light (which is typically unpolarized). This is why polarized sunglasses are able to reduce the glare off of the road and why polarized lenses can be used to reduce the glare off of water.

Circularly polarized light creates a spinning pattern, and it can be spinning either clockwise or counter-clockwise. To create circularly polarized light, you start with linearly polarized light and run it through a special filter that allows light to vibrate at two different speeds in two different (perpendicular) directions. But why would you want to convert linearly polarized light to circularly polarized light? Some electronics (TVs, monitors, smart phones) produce linearly polarized light, and research suggests that by adding an extra layer of polarizing filter to convert it to circularly polarized light, that it reduces eye strain.