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Why diamonds appear in car windows while wearing Polaroid sunglasses

I just heard this discussion on the air (8/10/2013), and the request for a physicist. I happen to be a physicist, at your service. The glass in side car windows is a special “tempered glass” that is made with internal stresses so that it crumbles into little round pieces when it breaks. The diamond pattern is put in the glass at time of manufacture and is normally invisible. However, Polaroid sunglasses can make it visible. Polarization refers to the direction in which light vibrates: side-to-side, up/down, or diagonally. Polarized sunglasses should be made to block horizontally vibrating light, such as light reflecting off of a road or water, while letting vertically vibrating light through. If you hold up a pair of sunglasses to the light while wearing another pair, you can see the light get dimmer and brighter as you rotate the first pair of sunglasses. Sunglasses held at right angles to each other should block all of the light. Some materials with internal stresses, like the car window, can rotate the polarization of light and act as the second polarizer. The amount of rotation depends on the amount of internal stress at different places in the glass, so the diamond pattern becomes visible when you wear Polaroid sunglasses.

The reason why Polaroid sunglasses sometimes couse headaches is because many pairs are made improperly. Light from the sky is naturally polarized. If the two lenses in your sunglasses aren’t polarized in the same direction, each eye gets a different amount of light and this can cause headaches. Fortunately, it’s easy to test new sunglasses in the store using the crossed-glasses technique.

Thanks, quantum mechanic! Good to have a physicist around when you need one. Plenty of auto physics mysteries for you to solve on this forum.

Quantum mechanic gave a good deal of good information, and I agree enthusiastically about bad polarized sunglasses. The question remains, why does it happen? My long-standing theory, for which I have no actual support, is that part of the tempering process includes heat treatment in an oven, during which the glass sits on a rack made to allow airflow over the glass. I think that the rack has a sort of open-grid pattern, and that something about the varying distance of different parts of the glass from the rack affects how the glass reflects light of different polarizations.
I am an engineer, not a physicist, although I do work with physicists. I would be glad to hear from someone who knows more about how the windows are actually made.

As an employee in a company that installs curtain wall and storefronts on buildings, here is what I have heard.

The stress patterns that you see are the result of the quenching process in creating tempered glass. The glass is quenched by a series of fans that blow air onto the glass to quickly cool it down. The patterns are a result of the locations of the fans, or air nozzles and where they blow on the glass.

Interesting stuff! As a photographer polarizer filters I have used were of 2 piece construction, allowing one to turn one piece of glass to help the blue of the sky jump out and reduce glare. I don’t recall seeing the patterns in car windows though, where is a physicist when you need one.

I too have seen this pattern, but not on all vehicle glass, only on the “solar ray” type of glass. This glass is very common today, probably in most cars sold today, and has a UV filter in it to help reduce solar gain inside the car on hot sunny days.

One thing I have not seen on cars though is tempered glass windshields. Vehicles require safety glass which is two planes of glass with a safety film between them. so if you see this pattern on a windshield, that kinda throws out the tempering part of the theory.

I used to see this pattern in the rear window of my Camry. I didn’t have or need polarized lenses to see this, but I only noticed it before sunset, in indirect light (shade). Come to think of it, isnt reflected light polarized?

I don’t see how one lens out of a pair of polarized lenses would be darker in direct sunlight, though. Since sunlight is omni-directional, each single-pane lens would block out the same quantity of light, regardless of orientation. Quality control could make two lenses of inconsistent tint, but that’s hardly a feature unique to polarization.

@meanjoe75fan, yes, sunlight itself is omnidirectional, but light waves of different types are not omnidirectional.


Look up Moire patterns in Wikipedia.

I’m a physicist too!

If you’ve never tried polarized sunglasses for driving, give them a try. It is easier to see bumps and other imperfections on the road surface.

Quantum Mechanic did a great job of answering most of the details to this question. I will just add a few other points. Tempered glass is created by a process where the glass is cooled in a manner which creates uneven temperature across the surface, which builds stresses into the glass as it cools. The more compression that is created, the stronger the glass is, but also the more fracture points are built into it, so when it does break, it will do so in smaller pieces. The idea behind this is to make the glass break into smaller shards which will be less dangerous and more pebbly, so that each piece can do less physical damage.

As mentioned, the process of tempering glass can involve the use of air nozzles, fans, or liquid to set up these compression stress points. As a result of this process, these compression stresses in the glass cause light to bend or refract differently within the glass itself. Normally, the light which enters the glass is highly scattered and random and so the result is these differences within the glass are not visible to us. However, by removing some of this scatter with polarizing lenses or filters, which also reducing the amount of reflection we experience, these patterns become more pronounced, so wearing polarized sunglasses amplifies the visibility of these stress compressions.

Certain lighting conditions naturally produce more highly polarized directional lighting, such as at sunrise and sunset, or when light is reflected from another surface. This can make the patterns more obvious. Polarized light is actually used in engineering testing as a way to reveal stresses in structures of transparent materials.

Lastly, someone suggested that front windshields, which are not tempered in the same manner and which have a plastic layer internal to them should not show these patterns. However, as it happens, the plastic layer internal to the glass also has even more extreme stresses within it due to the nature of thermal plastics films, and while the pattern may be more random, it can still show up as colored bands or rainbow patterns, almost looking like an oil slick on a water surface, which also involves polarized light and the thickness of the slick in different locations manifesting different colors, similar to what one sees on the surface of soap bubbles. For a good example of this phenomenon, take a look at commercial jet aircraft windows with polarized sunglasses the next time you fly.

Having seen these same patterns on office building windows, I always theorized they were a sign of some kind of transparent UV coating. I guess I was wrong.

Two phenomena are at work here, 1) two layers of glass with a very thin plastic bond between them. the plastic has weak polarizing properties, the polarizing angle depends on small thickness variations of the order of nanometers from place to place, thus mottled or blotchy patches of darkness appear and move around as you look at or rotate your polarized glasses; and/or 2) the glass has inbuilt stresses that optically produce “stress bi-refringence” which is an optical physics term for polarization angle change with internal stress from non uniform cooling down from the molten state. It is an inherent property of many glasses and crystals. These stresses are non uniform over the glass surface thus so the weak polarization angle varies over the surface making the glass appear blotchy. Most windscreens have the first phenomena , while the rest of the glass windows being single panes are blotchy from the second phenomena!

I can’t wear polarized sunglasses behind my motorcycle helmet’s face shield because it makes all kinds of psycodelic patterns and looks weird. I think that face shield is made out of polycarbonate plastic.

" @quantummechanic "
I’m sure impressed. I must admit even with a master’s in mathematics I had trouble following the explanation on first ( and subsequent) reading. Right now, my physics in practical use is limited to correctly adjusting the pole on a spinnaker. Now, how much is this condition aggravated by cataracts ? ;=)



Whenever I get a new eyeglass prescription, I order cheap sunglasses online with a non-polarized tint, but sometimes, when riding into the sun, I prefer the polarized lenses, so I have polarized clip-on lenses. Yes, it can make the road look kind of like some movie special effects, but I find it easy to adjust to the effect.

I’ve never noticed any odd patterns when wearing my polarized sunglasses.
BUT when I look at the LCD display, I need to tilt my head . Tilt one way and the display darkens unreadable… tilt the other way it lightes up so I can read it. ( just like holding two lenses over each other and twisting in opposite directions to prove if they’re polarized )

That’s because most of those displays work by manipulating the light polarization I think.