MLA Citation: Bloomfield, Louis A. "Question 1519"
How Everything Works 19 Oct 2017. 19 Oct 2017 <>.
1519. Why is a car's rear window put and kept under stress, and what has this to do with polarization? — BD, Leuven, Belgium
The rear window of a car is made of tempered glass — the glass is heated approximately to its softening temperature and then cooled abruptly to put its surface under compression, leaving its inside material under tension. That tempering process makes the glass extremely strong because its compressed surface is hard to tear. But once a tear does manage to propagate through the compressed surface layer into the tense heart of the glass, the entire window shreds itself in a process called dicing fracture — it tears itself into countless little cubes.

The stresses frozen into the tempered glass affect its polarizability and give it strange characteristics when exposed to the electromagnetic fields in light. This stressed glass tends to rotate polarizations of the light passing through it. As a result, you see odd reflections of the sky (skylight is polarized to some extent). Those polarization effects become immediately apparent when you wear polarizing sunglasses.

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