I’ve searched the internet, and, of course, here, but no luck, so I thought I’d ask the smartest group of people I know.
I recently got a new pair of sunglasses which are polarized, and I love them. I’ve noticed something strange though. Some windows of some cars, when viewed through the lenses appear to have a grid of spots, about 1-2 inches in diameter. It’s never the front windsheild, and it happens, I’d guess in 1 in 10 cars, including both of my own. Also, even less frequently, some car windsheilds appear to have a purple glow when veiwed with the glasses.
IANAWE (I Am Not A Windshield Expert), but I have noticed this with my polarized sunglasses too, and I think it has something to do with the car’s window glass tinting, and possibly they may also be polarized to a minor extent. Otherwise, why would we be seeing anything other than clear glass?
Anyone smarter than me care to refute?
Another interesting question would be: what do the “purple” and/or “splotched” car windows look like from the INSIDE with polarized glasses?
Have you noticed that the purple windshields seem to be mostly on those yuppie-vans with the green-house front windshields?
What you are seeing is an interference pattern resulting from the polarized sunglasses restricting incoming light which is itself polarized. This quite often happens with glass due to varying thickness - a useful if somewhat unintended effect of the glasses - you can see regions of stress in a pane of glass and observe how that stress changes as the glass is bent or stretched.
Other things to observe - any LCD display (such as a digital watch or pocket calculator). Look at these through your glasses, and then rotate the item - you should be able to tell the angle at which the polarization is the same (i.e. when it is brightest), and then at ninety degrees to that, you won’t see anything at all.
Also, look at the glare coming off the road, or sun reflected off the sea, and tilt your head to either side - the brightness will vary with angle.
You are seeing the stress pattern in the glass. Some materials, under certain conditions, can rotate the polarization of a light passing through. Glass under stress is one example: if you shine a vertically polarized light through an unstressed piece of glass it goes through unaffected, but if you apply a lot of shear force on the glass, the light coming out the other end will have its polarization axis tilted.
Now imagine that piece of glass sandwiched between two polarization filters, one oriented horizontally and one vertically. When there is no stress the light that goes through one filter will be blocked by the other. But when you apply stress to the glass, the polarization will rotate enough so that some light will come out of the other side. Such a device (setup) is called a polariscope. You can see an example here.
Automotive windows are tempered glass with stress patterns frozen in. They are also multi-layered. Reflection from transparent objects (e.g. at layer boundaries inside automotive glass) are polarized. So you have a setup where polarized light passes through a glass with a stress pattern, through a polarization filter (your sunglasses) and into your eyes.
Another nice effect occurs when I wear my polarized glasses and look through the visor of my motorbike helmet. The world becomes really strangely coloured.
Should be a similar effect as in the OP.
I assume these frozen stresses are put there so the glass will shatter nicely. If I’m going to shoot out a rear window with my B-B gun while wearing my polarized glasses, should I aim for the dots, the intersections of lines between the dots, or the border between the dots and the voids?
Pursuant to posts from Fuji and scr4, once I was in the Thermos factory in Connecticut (like 30 years ago) and saw the process for making the glass inserts. IIRC they had a way to do QA checks on the glass using polarized glasses or some kind of polarized viewer to look for stress flaws. Glass flaws that looked normal to the naked eye became amazingly obvious using this method.
