Found this:
Quote:
When viewed under particular lighting conditions, especially when viewed at a grazing angle, faint shadowy spots caused by the air quench process can often be seen in heat treated glass. These spots can be seen in transmission and in reflection, and at most viewing angles, other than directly facing the glass. They become very visible when polarizing sun glasses are worn. They are easily seen on a sunny day when the light comes from the blue sky or is reflected from clouds. They probably would not be visible on a grey, cloudy day. These spots are a normal function of properly tempered
glass and are mentioned in section 7.5 of ASTM C 1048 standard for heat treated glass. They are more visible when both lights of glass in an insulating glass (IG) unit are tempered. They are often seen in the sloping tempered back windows of cars, and in Asia they are very obvious in some small truck tempered front windshields.
Quench jets in a tempering furnace cannot cool the glass completely uniformly. As a result some areas are cooled faster than others resulting in differential shrinkage
which creates areas of different compressive stress. When glass is stressed in the quenching operation, with its corresponding very slight change in density, it becomes birefringant or polarizing, that is it partially blocks or passes polarized light. The greater the stress the greater will be the polarizing effect.
Vertically polarized light can easily pass through the vertically polarized sections of the glass to give bright spots but it is diminished when passing through other areas polarized at a different angle to give relatively darker spots. The spots are not an indication of absolute tempering level; they simply show areas of relatively more, or less, tempering stress. Given the nature of the quenching process it is physically impossible to quench a plate with absolute uniformity. The degree of tempering and the uniformity of tempering throughout the plate can be tested by other methods such as surface stress instruments or by examining the break pattern after fracture.
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More reading:
polarization of light: Definition and Much More from Answers.com
I also learned this is how 3D glasses work for 3D movies; one lense is polarized in one direction and the other lense is polarized 90 degrees from the first lense. This 90 degree difference only allows one eye to see what is meant for it, while blocking those images from the other eye, giving the effect of 3D.
So, auto window summary: The dots are there due to polarization to cut glare, but they can not entirely polarize a car's windows because if you were to drive with polarized sunglasses, you may not be able to see out of your window at all with them on due to the different anlges of polarization in the window and in your polarized sunglasses.
Class dismissed.