polarized lenses

[Beattie]

how come when you are wearing polarized sunglasses, you can see things like the spots in the back windshield of cars? And what are the spots?

 

[Viper GTS]

The spots are areas where the polarization of the vehicle's tint is 90° from the tint on your glasses, producing a dark spot.

Sunglasses 101

Viper GTS

 

[bmd]

For those too lazy to click:

Polarization

Light waves from the sun, or even from an artificial light source such as a light bulb, vibrate and radiate outward in all directions. Whether the light is transmitted, reflected, scattered or refracted, when its vibrations are aligned into one or more planes of direction, the light is said to be polarized. Polarization can occur either naturally or artificially. You can see an example of natural polarization every time you look at a lake. The reflected glare off the surface is the light that does not make it through the "filter" of the water, and is the reason why you often cannot see anything below the surface, even when the water is very clear.

Polarized filters are most commonly made of a chemical film applied to a transparent plastic or glass surface. The chemical compound used will typically be composed of molecules that naturally align in parallel relation to one another. When applied uniformly to the lens, the molecules create a microscopic filter that blocks any light not matching their alignment.

Most of the glare that causes you to wear sunglasses comes from horizontal surfaces, such as water or a highway. When light strikes a surface, the reflected waves are polarized to match the angle of that surface. So, a highly reflective horizontal surface, such as a lake, will produce a lot of horizontally polarized light. Therefore, the polarized lenses in sunglasses are fixed at an angle that only allows vertically polarized light to enter. You can see this for yourself by putting on a pair of polarized sunglasses and looking at a horizontal reflective surface, like the hood of a car. Slowly tilt your head to the right or left. You will notice that the glare off the surface brightens as you adjust the angle of your view.

A lot of sunglasses advertised as polarizing actually are not. There's a simple test you can perform before you buy them to make sure. Find a reflective surface, and hold the glasses so that you are viewing the surface through one of the lenses. Now slowly rotate the glasses to a 90-degree angle, and see if the reflective glare diminishes or increases. If the sunglasses are polarized, you will see a significant diminishing of the glare.

 

[Harvey]

That's why I hate polaraized shades. Even non-polarized glass can have some polarization, and I hate those random moire patterns, especially when they occur while looking through my windshield while driving.

 

[kgraeme]

Yeah, but polarized glasses used to be a fun way to play with TFT displays. Shift the polarization of the display and you can't see it without polarized glasses.

 

[Beattie]

Originally posted by: Viper GTS
The spots are areas where the polarization of the vehicle's tint is 90° from the tint on your glasses, producing a dark spot.

ok, but WHY are the windshields polarized like that? it seems to me that they should either be polarized completely or not at all.

 

[amnesiac]

Originally posted by: Beattie

Originally posted by: Viper GTS
The spots are areas where the polarization of the vehicle's tint is 90° from the tint on your glasses, producing a dark spot.

ok, but WHY are the windshields polarized like that? it seems to me that they should either be polarized completely or not at all.

Did you bother reading the article? Polarized materials are "fixed at an angle that only allows vertically polarized light to enter"
If something was "polarized completely" then you probably couldn't see through it.

 

[Beattie]

If the rear windshield was polarized vertically and the sunglasses were polarized vertically, there would be NO problem seeing through it. The problem is only introduced when the 2 polarized surfaces lenses are held at a 90 degree angle with respect to each other. Therefore that is NOT a good reason for the windshield to be spotted. Unless for some reason they wanted to polarize it horizontally, in which case, the driver, wearing vertically polarized lenses wouldn't be able to see through it.

 

[HomeBrewerDude]

I agree that your question as why you see a checkerboard pattern of alternating dark and ligh spots when looking at some tinted windows through polarized sunglassses remains unanswered.

I suppose that the tinting surface uniformly varies in how it polarizes light, but whether by design or defect i don't know.

Perhaps some window tinting film is manufactured in a way that causes the angle of polarization to systematically vary. Perhaps manufacturers do this on purpose so that a person in the car looking through the window would never be able to filter out all light (e.g., by rotating one's head while wearing polarized sunglasses).