Polarized sunglasses--those colorful patterns I see on some glass

I know that lightwaves normally vibrate in all directions, but a polarized filter will allow only those lightwaves to pass that are vibrating in a particular direction.

So, how does this cause those rainbow checkerboard-like patterns I see in some glass? I notice it most in auto windshields, but also just yesterday in the main door of the building where I work, as I was walking out last night.

That happens to other people? Oh thank goodness, I thought I was having old acid flashbacks.

The automobile windshield is a safety glass made from a layer of plastic sandwiched between two sheets of glass. The polarizing lenses show stress (and maybe adhesive) patterns from reflected (partially polarized) light coming through the windshield to your polarizing lenses - sort of like this, but not as intense.

I don’t think that’s what Spectre is referring to. It’s definitely more like a checkerboard than a moire pattern…although it may be related.

The colors are very similar, though not so intense as Savage said. As for the checkerboard, it’s really more a pattern in which the vertices of the squares stand out, which makes sense if it’s the adhesive I’m seeing, and if the adhesive is applied in rows and columns of small blobs before the two sheets are pressed together.

Years ago when the Lexus first came out, I had a pair of Polaroid brand polarized sunglasses. When I started seeing Lexuses (Lexii?) on the street, they all had a bright purple windshield. It wasn’t until I had to drive with no sunglasses one day that I realized that it wasn’t part of the design of the car…it was the polarized lenses. From my understanding of polarized lenses, I don’t know why it would’ve appeared purple though.

Also, why does the ocean look all weird with polarised sunnies?

Most reflected light contains some polarized component - the lower the angle of incidence (late afternoon, early morning) the higher the level of polarized content. That’s why polarized sunglasses work so well against road glare. That’s also why polarizing glasses are sold as “fishing” glasses - they reduce the glare off the water and allow you to see the fish (or old rubber boots, discarded tires and oil cans) more clearly.

That pattern I have noticed also requires a 3rd piece of glass. The sun glasses =1
the window=2, and either your car window, or the opposite piece of glass of the car you are looking at =3

“Many things look a little different, and maybe even a little odd, through polarized lenses. Drivers often see stress patterns in auto glass, plastic spectacle lenses and the like due to the birefringence (i.e., double refraction) of these materials. Stress can sometimes even be seen at the edges of polarized lenses themselves if they are imperfectly edged and mounted in a metal frame. Portions of the blue sky may darken through polarized lenses. This usually occurs with horizontal polarized surfaces, so patients will notice it only if they tilt their heads.”

from: http://www.2020mag.com/Issues/1998/Apr/lens.htm

Never thought I’d post the word birefringence. There it is.

The front glass on cars in laminated, as SavageNarce said. That keeps the windscreen from getting blown out by a pebble off a dump truck at 60mph. The other glass in a car is tempered glass. Tempered glass is designed to break into small, roundish pieces rather than sharp splinters. In houses and buildings, building codes now require tempered glass in the glass of outside doors. Anyway the tempering process leaves the glass with honeycomb-like patterns of stress built in, which you can see with polarized lenses.

You might also notice that the numbers on your watch and your car radio are hard to read through polarized sunglasses, until you tilt your head to the side. Another cute trick comes from overlaying one pair of polarized sunglasses on another. Rotate one lens 90 degrees, and the view darkens to nearly opaque.

That’s how I find out which sunglasses on the drugstore’s “tree” are polarized. Hold a pair of known polarized glasses sideways, and see which ones on the rack turn black. :slight_smile: