When driving with polarized sunglasses I noticed that the rear window shows a grid pattern: grid pattern on a rear window

Interestingly, the pattern is much more visible when viewing a reflective surface, like another car. I also observed the same pattern on some other cars' rear windows just when looking at them from behind. But it's always visible only with polarized glasses.

What is the cause of this? Is it related to safety features of car windows? Or to the fact that rear windows are (usually) tinted? And why does it show only with polarized sunglasses?

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    $\begingroup$ If you move your head around, does the grid appear to be stationary? $\endgroup$ – curiousStudent Aug 9 '16 at 12:10
  • $\begingroup$ @curiousStudent Yes, IIRC the grid appears to be stationary wrt the rear window. $\endgroup$ – Petr Pudlák Aug 9 '16 at 12:10
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    $\begingroup$ From Quora: "These spots are caused during the tempering process. They are intentional stresses which are created in the glass to make it stronger and to control the way it breaks in an accident. The result of this is that it breaks into thousands of small pieces instead of as few large knifelike pieces. The internal stresses can be seen under polarized light or through polarized glasses. The pattern of spots reflects the arrangement of heating elements or flames which are used to apply the heat. " $\endgroup$ – curiousStudent Aug 9 '16 at 12:13
  • $\begingroup$ for further info, also read up on Brewster's Angle $\endgroup$ – Jim Aug 9 '16 at 12:15
  • $\begingroup$ I think what @Jim means is that the light reflected from the surfaces like other car windows and the like becomes partially polarized owing to difference between the reflexion co-efficients for each polarization state (as described by the Fresnel equations) (Jim, hope I'm not misthinking you). The partial polarization then lets you see the effect noted by curiousStudent. I don't know anything about safety glass, so I don't feel I can answer but I'd wager a significant sum on the answer given by curiousStudent in the light of Jim's comment as being the answer, if I were the betting kind. $\endgroup$ – Selene Routley Aug 9 '16 at 12:41

Back car windows (and most side windows) are tempered for safety as you noted. The windshield is laminated.

This process is usually done by blowing cool air on the hot glass (after forming for example), this create surface stress in the glass, this energy is released when the glass break, fragmenting it.

What you see here are the marks due to the shape of the cool air nozzles. You will notice different pattern on different models due to the securities of the equipment used. The air nozzles make the cooling slightly inhomogeneous,thus the stress is can have a stronger direction making the glass slightly birefringent. Birefringence is well seen using polarized light and detector Photoelasticity

The light coming from the sky is partially polarized, light from reflection on dielectrics (like glass) also (depending on the angle) which explains your observations

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