Anyone who has driven, while wearing polarized sunglasses, knows how nifty and practical these lenses are for reducing glare and aiding in clarity.
My FIRST question is this: Why don’t automobile manufacturers make the windshield itself polarized?
This concept seems highly practical, desirable and probable, in my opinion. If you can put tint on a window, why not polarization? A windshield is a sheet of glass laminated between a transparent film, so it seems that this film could be easily made polarized prior to lamination.
My second question is: When you wear polarized glasses and look at a windshield, what are all of the “blue dots” we see in a criss cross pattern on the winshield?
Second question first, the blue dots are, IIRC, points of stress in the glass. Polarized light is used with clear plastic models of gears and such to examine stress points to determine where additional material or reinforcements are needed. Look at your windshield while someone presses on the glass and I’ll bet you can see the spots change with the new stress.
First question, because polarizing materials reduce the amount of transmitted light by a significant amount, driving at night or in other low light conditions would result in reduced visability. When I drive at night, I want all the light I can get.
To expand what GaryM said: Given an unpolarized light source, such as streetlights at night, polarized glass will transmit exactly 50% of the light. You don’t want this much loss when driving at night. It’d be even worse if you were also wearing polarized sunglasses: As long as you held your head straight, there wouldn’t be any more loss, but if you tilted your head slightly, it’d get even darker.
I think that some super-way-expensive luxury cars may have a system to electronically polarize the windshield on demand, like a giant LCD, but I can’t swear on it, and even if such systems do exist, they’d be more expensive than it’s worth.
Another thing about linear polarization. If you cross the lines of polarization at right angles, you’ll effectively block 100% of light throughput. So, windshields and sunglasses would require parallel lines of polarization.
Here’s a nifty java applet that generates sinusoidal and vector graphs for linear, circular and ellipitic polarization. You may vary the amplitudes by dragging the crests of the sine waves.
Chronos: EXACTLY 50% of the light? How such a precise figure? I don’t claim to be any kind of an expert on polarizing films, but I did get the high-school physics lectures and know that they work by passing the light rays through a plane of parallel “gates.” Is the “opaque” portion (which blocks a ray) exactly as wide as the transparent? Or is your claim based on the notion that exactly 50% of any sample will randomly be perpendicular to the gates? Seems like a few more than 50% would be parallel - or at least close enough - and hence make it through.
I humbly offer you a reductio ad absurdum: If polarizing lenses did not block 50 percent of the light attempting to pass through them, then thay would admit some lesser percentage, yes? Let’s call it 48 percent.
Now: take two polarizing lenses, and turn them so that their polarizing planes are perpendicular to one another. If, as we have supposed, they each blocked 48 percent of the light attempting to pass through them, the total blocked light would equal 96 percent of the light hitting the lenses.
Thus, 4 percent of that light would be conveyed through the lenses. Now, 4 percent is about as much light as passes through extremely dark sunglasses. Can you see through the two lenses at all? No? Then they must each be blocking 50 percent of the light attempting to pass through them.
Q.E.D. (Latin for “Hah! Gotta jump back and kiss myself!” as exclaimed by James Brown.)
Okay, I admit that it could work out to 49.993 percent blockage for one lens, but Occam’s razor should shave that .007 percent differential down to a nice, clean zero.
Actually, that very low percentage transfer is sort of what I was talking about. I’ve tried this a buncha few times, and have never found a set that perfectly blocked the light. Some have been close, but if held close to the eye you could still see light. So take your Occam’s razor elsewhere , I’m talking real world.
And from where do you cite 4% transmission for dark shades?
The figure of exactly 50% is for a perfect polarizer-- Apologies if I didn’t make that clearer before. A photon polarized at an angle of exactly 0[sup]o[/sup] or 180[sup]o[/sup] is guaranteed to pass through, one at 90[sup]o[/sup] or 270[sup]o[/sup] is guaranteed to not pass through, and one at an angle in between has a certain probability to pass through-- Assume unpolarized light, and add them all up, it works out to a 50% for any given random photon. The thing is, though, after the first polarizer, all of the light that comes out is perfectly polarized parallel to the first filter, even the photons that started off at an angle. If the second polarizer is lined up the same as the first one, there’d be no additional effect, and if it was at 90[sup]o[/sup], all of the photons would be blocked. Intermediate angles would, of course, result in some additional photons to be blocked, but not all.
Polarized sunglasses are more clever than you realize. They’re designed to cut down a lot of the glare without being too dark to see through. It turns out that when randomly-polarized light (like sunlight) reflects off a surface at a certain angle, it becomes uniformly polarized. IIRC, the light polarizes vertically, so sunglasses with horizontally polarized lenses will block the glare. And that’s what you want when you’re driving, the sunlight overhead isn’t as much of a problem as all the bright reflections in your line of vision.
I remember working through the math for this back in a college physics course. It has to do with interference between the electrical and magnetic waves that make up an electro-magnetic wave. I'll dig through my textbooks and see if I can find something to back me up on this.
You can try an experiment to test this. Go out to a parking lot in late afternoon, when the glare should be at its worst. Take the sunglasses off your face a few inches and look through them as you turn them 90 degrees. I've never thought to try this myself, but I'd be curious how it turns out.
RobotArm is correct. Light becomes polarized when it is reflected. The reason the glasses work so well is that the windshield is tilted forward. Any light reflected from the dashboard up to the glass and towards the observer will have most of the light polarized in the horizontal plane. If you wear glasses polarized in the vertial plane, you miss those reflections. If you make the windshield itself polarized, it would have no effect on light reflected from inside the car. Try twisting the glasses in front of your face while looking forward in the car. You’ll see that when the glasses are up-and-down, you can see the objects on the dashboard quite well.
IIRC, Ford once considered polarizing the windshield and the headlights in the opposite directions to lower the glare of oncoming vehicles at night. My grandfather told me that Mr. Land and Mr. Ford were unable to come to terms.
Ed Land, inventor of the sheet polarizer and founder of Polaroid Corporation, wanted to do Precisely what you suggest, and he lobbied long and hard for it. It still didn’t happen.
As people above have stated, though, you throw away at least 50% of the light when you “strain” the light through a polarizer. In fact, you usually lose more. Look at the spec sheets of polarizers – most are far below the theoretical maximum. The transmission varies considerably with wavelength, too. In any case, inevitably tossing out over half your light, especially at night, is a dangerous proposition.YOU try driving at night with your polarized shades on and see how YOU like it. Don’t do it around me.
A quick note on polarization directions – play around with those polarized 3D movie glasses some time. The polarization directions don’t run horizontal and vertical, but at plus and minus 45 degrees. I expect that they’d do the same with windscreen polarizers and automobile headlamps.
I can’t say that I know this for sure, but I always assumed that those spots were the glue which is used to affix the tint to the window. IIRC you don’t see these spots on non-tinted windows.
Mrs. O and I got two free pair of polarized sunglasses from the clinic at which she had her eye surgery - neither of us have seen those blue crisscross dots on the windshield as mentioned in the OP. We don’t have tinted windows, either.
I vaguely remember how all this worked from physics in high school, but more important to me is how well the damn things work. It’s an absolutely great feeling to be able to go out at noon and not squint like your eyeballs are trying to pop out of your head.
Tell me about it, polarized sunglasses are a necessity here in South Florida. My eyes are light-sensitive to begin with, especially with my contact lenses. I’ve been caught at times without my sunglasses outside on a sunny day where I literally could not open my eyes. When this happens, I usually open my eyes for a split-second every few seconds just to get my bearings. I find that the time when my eyes are the most sensitive is when I first put on my contacts in the morning.
Do they even make cars with tinted windshields (i.e., the front window)? I know they do it for the sides and rear, but I had thought that tinting the front was considered a safety hazard.
Often when the sun is high in the sky you don’t need the polarized glasses to see the dots in the -rear- auto glass. I see the windshield dots often in the rear windows of cars; I don’t recall ever seeing them in the front windshield or the side windows. And it occurs to me that although you can see the dots directly, you tend to be looking through one window - at the reflection of another window. I don’t know what you see if you look at another rear windshield with dots, through your own rear windshield. Hmmm . . . .
I also remember that there’s a fish whose eyes are polarized 90 degrees out of phase, and that this helped it see better somehow in the murky depths in which it lived. I don’t recall if it only works underwater or not. And that visors for high-altitude pilots (such as U-2 & SR-71) were polarized similar to this, that is, out-of-phase though I dunno if it was 90 degrees. The polarization helped to be able to see clouds, or moisture, or something.
IIRC, Aston Martin had one car that had LCD variable tint on the windows, but it may not have been a regular model and the regular models cost a fortune. I don’t remember if it included the front windshield or not, and nobody in my neighborhood owns one to inspect. Aston Martin takes special pride in being the car maker that people who own Ferrari’s can’t afford. - MC