Patterns on Back Windows

What exactly are those stupid patterns on the rear windows of cars? You know what I mean, it looks like there’s a pattern of blocks that needs to be washed off, but you can never get them off. Is this a special coating of some sort? It seems like you can see them better from the outside than the inside, and it seems like newer cars have them rather than older cars…

Those are for defrosting the window. They’re little wires embedded in the glass that heat up.

if you’re seeing what I think you’re seeing, the answers to the second question of this post should help.

Ok, if they are stress marks or indicators, why do I only see them on the rear windows and never the front?

that wasn’t referring to the lines; it was instead talking about the blue dots you see when you look through polarized lenses. (I think) The lines are indeed defroster wires.

Are you talking about the fuzzy grid of patches? Like a checkerboard, with the patches each an inch or two wide?

Yes, it’s an accidental stress pattern frozen into the glass. It’s revealed by crossed polarizing filters. Our unaided eyes often see them because there are two polarizing filters which occur naturally: the blue sky, and the reflection from internal glass surfaces.

Little-known fact: a broad band in the blue sky is a natural source of polarized light. Light from the sky near the sun isn’t polarized, and neither is light from the point on the sky directly opposite the sun. But between these two “poles” is a broad fuzzy “equator.” That band looks much darker blue than the rest of the sky. It’s also polarized. Look at it through polaroid sunglasses, then rotate them rapidly back and forth, and the sky will flash dark and light. (Right at sundown this polarized sky is straight up, directly overhead.)

Another little-known fact: if you bounce some light off a piece of glass (not a mirror), then the light will become partially polarized. This effect is maximum when the angle of reflection is Brewster’s angle.

Combine the two facts and you get two polarizers with a hunk of stressed glass between them. You’ll only see those fuzzy patches when the plate of glass is reflecting the dark blue part of the sky, and when it’s also tilted right so the light reflected from the glass is polarized at a different angle from the sky polarized light.

But WHY is there a fuzzy grid of stress in the rear windows of cars? I’m not sure. I think it has to do with the rear window being tempered glass. If you chip your front window, you see a small crack. But if you chip your rear or side windows, the glass “explodes” into shards shaped like tiny ice cubes.

Maybe the grid pattern is from the cold metal rollers that the hot glass plate first rolled across during manufacture?

bbeaty: That’s the best explination I’ve heard so far. Thanks!

I have a rather large nit to pick here. All the windows on a modern car are tempered glass. If the windshield were not tempered glass then when your head went thru it, it could easily cut your head off on the large jagged edges.
This was not always the case. Volvo started using tempered windshileds right after the war on the 444. This was either the first use of TG for a windshield or one of.

The difference between a windshield and a rear window is the windshield is actually two pieces of tempered glass with a plastic sheet in between to prevent your head from going through the glass in the event of an accident. The rear window is a single piece of tempered glass with no liner. side windows are usually a single sheet of tempered glass.
However some car makers are offering a security glass option where the side windows, and sometimes the rear window also use a plastic sandwich. This prevents someone from throwing a brick through the window and stealing a purse off the seat. The plastic sheet doesn’t break it streaches. This really pisses off the crooks. :slight_smile:

Getting back to the OP a rear window is more likely to be tinted, and thus acts like a surface mirror, this makes the pattern more visible than it would be on a windshield. However I have seen this patterns on windshields, but never as apparent as it is on rear windows.

On and the reason the front window doesn’t explode like the rear one is the plastic sheet in the middle, it holds everything together.