Polarized 5/2/21

Science with Richard Bleil

The weather outside has been beautiful. Truly, the week has been great for mini-trips and running errands.

A few days ago, while on the road, I put on my new polarized sunglasses. Polarized sunglasses are specifically designed to cut back on glare by taking advantage of polarized light.

As you are probably already aware, light is a wave. Actually a “photon” of light is like a little packet of light energy and is a wave. That light moves in a straight line from the source of the light to where it strikes, such as your eye. The question is, though, how does the light wave orient itself along that line of travel?

The light wave could be going up and down along the path, or side to side, or anywhere in between. There are countless photons (actually they can be counted; it’s a function of the intensity of the light) traveling along that pathway, and they are oriented in every possible direction. Think of the light as sheets of paper in a book. If you can imagine opening the book, say in space, all the way around so that the front cover touches the back, you can imagine that the pages will basically create a circle all the way from one cover to the other. Every page is going in the same direction (defined by the spine of the book) but the planes defined by the pages are oriented in every possible way.

A polarized lens blocks out every plane of light, except for one. In polarized sunglasses, the only plane of light allowed to pass is perpendicular to the ground. If the light plane is not going straight up and down, it’s simply filtered out. There are plenty of photons in that one plane that you can still see, but the light reflecting off of the back of the vehicle in front of you is largely light along the plane parallel to the ground. This is glare, excess light reflected from the vehicle’s surface.

A polarized lens, then, blocks out this plane of light. Since it only allows perpendicular plane light to pass, the parallel plane, the light from glare, is blocked. This is how polarized lenses work. Here is a fun side effect of this, though. If you take two polarized lenses (say from two polarized sunglasses, or two lenses removed from the frame of polarized sunglasses) and look through them both, the lenses will darken or lighten as you turn one of them. When aligned, they both block the parallel light, but when you turn a lens, one will block parallel and the other will block the perpendicular. Aside from light bleed, it will block all of the light.

Driving on a warm sunny day, I learned something fascinating as I put on my polarized sunglasses. In all of my vehicle windows, except for the front, there appeared dark and light spots, not unlike the spots of a leopard. When I stopped, I performed the experiment. I looked through the lens and turned the sunglasses. Sure enough, the dark spots shifted as I turned the polarized sunglasses.

As it turns out, the car windows are indeed polarized. Because of the pattern, I do not believe this is intentional, but it’s more likely to have been an artifact of the manufacturing process. The front windshield must have been manufactured differently. I’m guessing the front windshield was more carefully manufactured to be higher quality. I do know that windshields are supposedly shatter resistant, but side windows are not. The idea being, no doubt, that if the windshield shatters then flying shards into the fact are a particular hazard, but if you’re stuck in your vehicle, the side windows (and back) need to be capable of being shattered for escape.

The crystalline structure of the glass, then, aligns themselves into long chains. These chains are apparently polar, so some spots in the window is blocking perpendicular plane light, and others is blocking parallel plane light. As I write this, I realize that this might, indeed, be intentional. As it turns out, these different regions make the glass non-homogeneous, even if it’s only visible through polarized lenses. The interface between these differing regions of glass, some blocking parallel plane light and some blocking perpendicular plane light, will be areas of weakness in the crystal structure of the glass. These weak spots should make the glass easier to break.

As a side note, if you ever are required to break glass, don’t strike it in the center of the glass. In the center, the glass has more freedom of motion, and it’s incredibly difficult to break the glass because it simply moves when struck. Instead, strike near the base, especially near a corner of the glass like near the rear-view mirror (or equivalent location on other windows). This point of contact with the car holds the glass more rigidly, so when you strike it the glass is less likely to absorb the impact by bending making it more likely to break.


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