Glass and Sunburn

[richardm]

I was talking to an Australian friend a few days ago, and she reckoned that on a long trip across Sunny Parts of the country she'd got quite a bad sunburn. Not because she was hanging out of the open window, but just because the sun was so relentless through the glass.

I was a bit surprised about this, because I had always thought that you couldn't tan through glass. I assumed that the stuff that causes tanning is the same stuff that causes burning - perhaps that's a crucial error.

I also recalled reading here about one of the A-bomb developers (Oppenheimer?) watching the first detonation through a truck windscreen, knowing that the nasties in the blast would be stopped by the glass. I had also assumed that those same nasties were the things that damage skin.

Can anyone fill in the gaps in my knowledge and save me from offending Australians in future?

[Soapy Sam]

I suspect it would have been Feynman. Sounds his style.

There's glass and glass. Different mix, different properties. I suppose some may screen out less UV than others. I know you can't use a polarising filter on a camera through modern coach windows, because the heat stress patterns show up.

I would have thought modern double glazed coach windows would prevent sunburn, but they don't stop all heat by any means. If you sit by a sunlit window in Oz for hours , you might well get burned just by reradiated infra red.

[Brian the Snail]

Yep, it was Feynman.

Usually glass absorbs UV, especially the shorter wavelength (and therefore more energetic) UVB rays that are usually responsible for sunburn. But the amount absorbed depends upon the thickness and type of glass used. So while it is unusual to get sunburnt through glass, given the right conditions and for an extended period I don't think it would be impossible. So I think that you are correct in general, but it could be that your friend was also correct and that she was just unlucky.

[Iconoclast]

a) If you drive cross country all day all Australia in Summer you end up with one sunburnt arm.

b) Feynman was looking through the polarising strip along the top of the truck's windscreen.

[Ladewig]

Originally posted by Iconoclast

b) Feynman was looking through the polarising strip along the top of the truck's windscreen.

Do you have a citation for that? I am having a hard time understanding why one would polarize the top of a 1940's windscreen instead of the bottom.

[Iconoclast]

Originally posted by Ladewig
Do you have a citation for that? I am having a hard time understanding why one would polarize the top of a 1940's windscreen instead of the bottom. Sure, "The Pleasure of FindingThings Out", 1999, pp88-89.

"So then I figured the only thing that could really hurt your eyes -- bright lights can never hurt your eyes -- it's ultraviolet light that does that. So I got behind a truck windshield, so the ultraviolet can't go through glass, so that would be safe, and so I could see the damn thing."

So, basically, I dunno what the hell I'm talking about.

The strip along the top of a (modern) windscreen is tinted, NOT polarized, it's a circular area in the driver's field of view which is polarized. I don't believe it would have been possible to have included more factual errors in two lines of text.

[Soapy Sam]

You could have said "motorcycle" instead of "truck" ?
:D

[Ladewig]

Originally posted by Iconoclast
The strip along the top of a (modern) windscreen is tinted, NOT polarized, it's a circular area in the driver's field of view which is polarized.

Part of modern windscreens are polarized?
I'm not sure how to ask this without sound like a bit of a jerk but, do you have a citation for that one?

[Soapy Sam]

Thanks, Ladewig. I was trying to think of a nice way to frame the same question. I know modern toughened windscreens show stress patterns through polarising filters, but I never heard of them being deliberately polarised.

[richardm]

Thanks, all!

I've never heard of a polarised windscreen either. I wonder why you would do such a thing - seems to me it's got a good chance of really annoying people who wear polarising sunglasses!

[Orangutan]

There's a couple of easy way to find out if a medium (in this case the glass) is polarised.

A.
1) Take your polarised sunglasses and an unpolarised light source like a flash light. (You have to use this as there are lots of polarised sorces in a brightly lit natural scene, the blue sky, the surface of the road, the reflections from other cars windscreens and surfaces. That's why polarised glasses are so great for drivers).

2) put the sunglasses on and observer the flash light through the screen, Now tip your head through 90 degrees. Does it go dark?
Yes the glass is polarizing, no it is not.

B)
For this you need an already polarised light source. Many of you probably have one that you may not realise it. You'r mobile phone! If it has an LCD (color or gray) screen you can use it as a polarised source. If you don't have this maybe you can find an LCD calculator to use somewhere.

1) get a friend to hold the phone display against the glass. (Make sure the display is on if it is black without a backlight, grey lcd don't need to be on.)
2) Rotate the display through 90 degrees.
3) does it go dark while observing it through the glass?
Yes the glass is polarizing, no it is not.

C)
For fun, If you havn't looked through polarised sunglasses at you phone display on its side you should try it. Its a good test when someone trys to tell you the glasses they are selling you are polarised.


O.
:)

[Iconoclast]

So... I hope nobody minds if I just skulk away now, I'm having a bit of a shocker today.

[Frinkiak7]

Most of the window tinting films that I've used have been polarized, but I don't think I've ever come across auto glass that was polarized from the factory. I guess it could vary from manufacturer to manufacturer, though. I can't find any reference to polarization on PPG's website.

[Soapy Sam]

Well, if it was really plane polarised, a rotating polarised camera filter would black out twice as it rotated through 360 deg.
It doesn't, but there are visible birefringence colours at certain positions of the filter on some car and bus windows.
I suppose the partial polarisation is an artifact of the toughening process.

If you shine a really bright light through, you can just make out a shivering wreck of an iconoclast in the bottom right corner.
Come out sir! All is forgiven!
:D

[TeaBag420]

I'd stilll be interested in the citation of a source for the original assertion that "it's a circular area in the driver's field of view which is polarized."

Extra credit for some information about the manufacturing process for such windshields.
:bs:

[jj]

I've used polarized sunglasses a lot, and camera lenses with both linear and circular poliarizers. I've looked out of cars with both.

(btw, "circular" is just a linear polarizer on the OUTside with a differential wave plate afterwards so that the metering and audofocus systems on a modern camera work)

I have seen all sorts of stress polarization, etc, in window glasses, not just in cars, but I have yet to see anything that was actually, really polarized. I've seen some polarization due to angle of incidence, and polarization in skylight, etc.

What's interesting is that when I was taking shots of Riverside Geyser at Yellowstone, wearing polarized glasses, I was completely uanware of the rainbow that the water drops were forming. My camera was, however, aware enough to capture the effect quite nicely despite my lack of knowledge.

I went back w/o sunglasses, sure enough, the sunglasses were blocking my perception of the rainbow effect.

[Iconoclast]

Originally posted by Soapy Sam
If you shine a really bright light through, you can just make out a shivering wreck of an iconoclast in the bottom right corner.
Come out sir! All is forgiven!
:D I was told quite specifically about this polarized area on the windscreen way back when I was in high school. What's really weird is that Google's web crawlers don't seem to have caught up yet...

[TeaBag420]

Originally posted by Iconoclast
I was told quite specifically about this polarized area on the windscreen way back when I was in high school. What's really weird is that Google's web crawlers don't seem to have caught up yet...
And whoever told that gets this:

:bs:

[Soapy Sam]

I know many boats (seagoing jobs, not rowing boats) have a circular glass disc on the bridge windows which is spun by a motor. Inertia throws water off and keeps it clear. Waves tend to lift window wipers off the surface.

That's the only circular glass thing I've seen on any windshield. I think jj' s definition of circular (polariser) is describing a rather different meaning of the same word.
Actually, a big centrally mounted stick-on polarising disc that adhered to the inside of a windshield and rotated, might be a rather neat idea when driving in bright sunshine.

Bags I the patent.

[CaveDave]

Originally posted by Iconoclast
I was told quite specifically about this polarized area on the windscreen way back when I was in high school.

I was told quite specifically that open (non-positraction) differentials only sent power to the right-rear wheel and that an automotive storage battery placed on the ground would have the power "sucked" out. Both tales came from several authoritative sources, and both were so false they weren't even wrong. ;)

Question authority.

Dave

[Soapy Sam]

Originally posted by CaveDave
I was told quite specifically that open (non-positraction) differentials only sent power to the right-rear wheel and that an automotive storage battery placed on the ground would have the power "sucked" out. Both tales came from several authoritative sources, and both were so false they weren't even wrong. ;)

Question authority.

Dave

Are you absolutely sure they said "placed on the ground" , not "connected to ground"?

[Bearguin]

Originally posted by CaveDave
I was told quite specifically that open (non-positraction) differentials only sent power to the right-rear wheel and that an automotive storage battery placed on the ground would have the power "sucked" out. Both tales came from several authoritative sources, and both were so false they weren't even wrong. ;)

Question authority.

Dave

Open diffs send power to the wheel with the least resistence. If the wheels are comparable in resistance, both get power. Pick one wheel up off the ground (or put it in slick mud / snow) and it will get all the power with no power going to the other wheel.

Old car batteries (50's vintage) would lose their charge if place on concrete. It was a result of poor manufacturing and does not apply to batteries from today's era.

Cartalk Explanation (http://www.cartalk.com/content/columns/Archive/1999/November/06.html)

[Ladewig]

Originally posted by Bearguin
.

Old car batteries (50's vintage) would lose their charge if place on concrete. It was a result of poor manufacturing and does not apply to batteries from today's era.

Cartalk Explanation (http://www.cartalk.com/content/columns/Archive/1999/November/06.html)

But if the battery were leaking enough that contact with damp concrete would drain the battery, wouldn't putting the battery down on any surface drain the battery? It would leak until there was a circuit and then the circuit would drain the battery.

What am I missing here?

[Bearguin]

Originally posted by Ladewig
But if the battery were leaking enough that contact with damp concrete would drain the battery, wouldn't putting the battery down on any surface drain the battery? It would leak until there was a circuit and then the circuit would drain the battery.

What am I missing here?

I think that putting it on a shelf would reduce the likliehood of losing the charge. And they were talking about small amounts of seepage, not enough to really notice but enough to create a small current and drain the battery. Not sure if damp concrete was really critical, just a conductive path to ground.

I was really just pointing out that it wasn't wrong advice at one time, and that it no longer applies.

And I had a link to back that up ;)

[TeaBag420]

Originally posted by Bearguin
I think that putting it on a shelf would reduce the likliehood of losing the charge. And they were talking about small amounts of seepage, not enough to really notice but enough to create a small current and drain the battery. Not sure if damp concrete was really critical, just a conductive path to ground.

I was really just pointing out that it wasn't wrong advice at one time, and that it no longer applies.

And I had a link to back that up ;)

I guess that's why batteries in engine compartments sit on metal trays. No chance of losing a charge through that.

[BillHoyt]

Originally posted by TeaBag420
I guess that's why batteries in engine compartments sit on metal trays. No chance of losing a charge through that.

Really? Your car has metal wheels like a train?


Oy.

[69dodge]

Originally posted by BillHoyt
Really? Your car has metal wheels like a train?

Oy.Wheels? What have wheels to do with anything?

If you leave your headlights on, your battery will run down. Electricity flows through the battery and through the headlights. The ground beneath the car's wheels has nothing to do with it.

[CaveDave]

Quote from Soapy Sam
Are you absolutely sure they said "placed on the ground" , not "connected to ground"?

[CaveDave]

Sorry for that premature post.

Quote from Soapy Sam
Are you absolutely sure they said "placed on the ground" , not "connected to ground"?
Yes. They claimed you had to put wood underneath to stop the electricity.

Quote from Bearguin
Old car batteries (50's vintage) would lose their charge if place on concrete. It was a result of poor manufacturing and does not apply to batteries from today's era.
This myth REALLY came from the fact that batteries self-discharge over time, and then sulphation sets in ruining the plate's ability to take a charge. Most of the time, a battery was put on the ground because something was already wrong with it, and set-aside batteries usually stay for a long time without attention.
Metal trays would have been worse than concrete or dirt if the myth were true, and 'the ground' is not 'electrical ground' unless a connection is made.

Quote from Bearguin
Open diffs send power to the wheel with the least resistence. If the wheels are comparable in resistance, both get power. Pick one wheel up off the ground (or put it in slick mud / snow) and it will get all the power with no power going to the other wheel.
The effect happened because driveshaft torque planted the left tire and lifted the right and allowed it to slip.

Dave

[Art Vandelay]

Originally posted by Iconoclast
Sure, "The Pleasure of FindingThings Out", 1999, pp88-89.

"So then I figured the only thing that could really hurt your eyes -- bright lights can never hurt your eyes -- it's ultraviolet light that does that. So I got behind a truck windshield, so the ultraviolet can't go through glass, so that would be safe, and so I could see the damn thing." I'm pretty sure that eyes can be hurt by non-UV light. And UV can get through glass.

richardm never heard of a polarised windscreen either. I wonder why you would do such a thing - seems to me it's got a good chance of really annoying people who wear polarising sunglasses!That seems like an odd question to ask. Why question windshield polarization, but not sunglasses? Light that bounces off the ground tends to get polarized, so polarized glass cuts down on direct sunlight without having much impact on other light.

BillHoytReally? Your car has metal wheels like a train?
What are yours made of? Plastic?

CaveDaveMetal trays would have been worse than concrete or dirt if the myth were true, and 'the ground' is not 'electrical ground' unless a connection is made.
It can be.

[anor277]

Art V wrote
I'm pretty sure that eyes can be hurt by non-UV light. And UV can get through glass.

This is absolutely correct. In UV-visible spectroscopy, the cell (i.e. the container holding the species that is UV-visible active) is often made of glass or quartz (a UV-visible "window" down to about 200 nm). On the other hand an IR cell is never made from glass because it absorbs IR radiation; for this reason IR cells are made from NaCl or CaF2, which materials are non-IR active.

[richardm]

Originally posted by Art Vandelay
richardmThat seems like an odd question to ask. Why question windshield polarization, but not sunglasses? Light that bounces off the ground tends to get polarized, so polarized glass cuts down on direct sunlight without having much impact on other light.


Because lots of people have sunglasses that are polarised. If you also have a polarised windscreen then there is a chance that the two will give you zero visibility. So I would be surprised to hear that such a thing existed, purely because a few seconds of surprised blindness on a motorway could be fatal.

[richardm]

Originally posted by BillHoyt
Really? Your car has metal wheels like a train?


Don't rubber car tyres contain enough carbon to be a good conductor? This is why being in a car that's struck by lightning doesn't kill you - it's a Faraday cage, with the current going straight to ground around the outside of the car and the tyres.

[Orangutan]

I don't know if that's the case or not, but having enough potential to break down the resistence of a few thousand feet of air, I don't suspect an extra few inches from the base of your car to the ground would phase it much.

It's raining at the moment so I can't go out and test my tyres with my multimeter to test thier resistance.

The Faraday cage effect would still be true though.

:)

[Rob Lister]

Originally posted by CaveDave
Sorry for that premature post.

This myth REALLY came from the fact that batteries self-discharge over time, and then sulphation sets in ruining the plate's ability to take a charge. Most of the time, a battery was put on the ground because something was already wrong with it, and set-aside batteries usually stay for a long time without attention.
Metal trays would have been worse than concrete or dirt if the myth were true, and 'the ground' is not 'electrical ground' unless a connection is made.

Quote from Bearguin

The effect happened because driveshaft torque planted the left tire and lifted the right and allowed it to slip.

Dave

I've heard of this effect also but my understanding was that it had to do with the temperature difference between the top and bottom of the battery. The difference was enough to cause a strictly internal flow of voltage and thus discharge the battery more quickly. I remember who told me too! It was my eleventh grade 'shop' teacher...Mr. Garrett I think. He got fired because Tim...some other student or whatever...cut their finger off on a surface plainar/planer/whatever.

I suppose that should make is data inaccurate...arguments from authority being what they are.

[Ziggurat]

Originally posted by richardm
Don't rubber car tyres contain enough carbon to be a good conductor? This is why being in a car that's struck by lightning doesn't kill you - it's a Faraday cage, with the current going straight to ground around the outside of the car and the tyres.

Diamond contains a lot of carbon too, and it's very much an insulator. You can't judge the electrical resistivity solely on what elements if contains, and car tires are definitely insulating. But it doesn't mater: as mentioned above, if the lightning can ionize the air between your car and the clouds above, it's not going to be very hard to ionize the few inches of air between your car and the road. None of that affects the fact that, yes, your car will act as a Farraday cage, and current will flow through the skin of the car, not the passenger area.

[davefoc]

Originally posted by richardm
Don't rubber car tyres contain enough carbon to be a good conductor? This is why being in a car that's struck by lightning doesn't kill you - it's a Faraday cage, with the current going straight to ground around the outside of the car and the tyres.

exactly, that's why when lightning strikes an airplane everybody dies on the inside. Some airplanes solve this problem by dragging long wires that drag across the ground as they fly.

[Art Vandelay]

Originally posted by Soapy Sam
I would have thought modern double glazed coach windows would prevent sunburn, but they don't stop all heat by any means. If you sit by a sunlit window in Oz for hours , you might well get burned just by reradiated infra red. Do you mean literally "burned", or sunburned? Because infrared doesn't, AFAIK, cause sunburns.

richardm
Becauselots of people have sunglasses that are polarised. If you also have a polarised windscreen then there is a chance that the two will give you zero visibility. So I would be surprised to hear that such a thing existed, purely because a few seconds of surprised blindness on a motorway could be fatal.But why question windshields specifically? Wouldn't it make just as much sense to ask "given that there are polarized windshields, why polarize sunglasses"? And actually, the chance getting zero visibility is rather low. If you ever find yourself trying to operate a car while laying on your side, I think your sunglasses would be the least of your concerns.

[CaveDave]

Originally posted by Rob Lister
I've heard of this effect also but my understanding was that it had to do with the temperature difference between the top and bottom of the battery. The difference was enough to cause a strictly internal flow of voltage and thus discharge the battery more quickly. I remember who told me too! It was my eleventh grade 'shop' teacher...Mr. Garrett I think. He got fired because Tim...some other student or whatever...cut their finger off on a surface plainar/planer/whatever.
Not too long ago, I had the need for detailed lead/acid battery charging information, and ran accross several very detailed descriptions of the self-discharge problem, and none of them mentioned temp gradient in the cell, just the overall temp (I would think temps would equalize pretty quickly, anyway). Google will give more data than most people can digest. :)

Originally posted by Rob Lister
I suppose that should make is data inaccurate...arguments from authority being what they are. :D
Indeed. ;)

[richardm]

Originally posted by Art Vandelay
But why question windshields specifically? Wouldn't it make just as much sense to ask "given that there are polarized windshields, why polarize sunglasses"?

I'm guessing that it's more expensive to make polarised glass than non-polarised. Consequently you'd be much more likely to encounter small bits (in sunglasses) than in large sheets (in windscreens)

If you ever find yourself trying to operate a car while laying on your side, I think your sunglasses would be the least of your concerns.

That's assuming that your sunglasses are polarised in the same direction as your windscreen, of course...

Originally posted by Ziggurat (and by Orangutan, a bit)
Diamond contains a lot of carbon too, and it's very much an insulator. You can't judge the electrical resistivity solely on what elements if contains, and car tires are definitely insulating. But it doesn't mater: as mentioned above, if the lightning can ionize the air between your car and the clouds above, it's not going to be very hard to ionize the few inches of air between your car and the road.

Right, thanks! I'll stop telling people that, then (wonder where I picked that idea up from?)

[Soapy Sam]

ART- I mean burned. Sunburn is a burn like any other, so far as I know. The cause is UV exposure, but it's still a burn.

I've certainly been burned by the metal around a car window. It actually left a red burn mark which took weeks to fade. That was a brief contact with metal. I can't see any clear reason why a much longer exposure to radiant heat might not have a more distributed, but essentially similar effect.

[Art Vandelay]

Originally posted by Soapy Sam
ART- I mean burned. Sunburn is a burn like any other, so far as I know. The cause is UV exposure, but it's still a burn.No, a burn is an injury caused by heat. A sunburn is not caused by heat, and it isn't really an injury; the trigger is UV exposure, but the cause is the body's reaction to it.

I can't see any clear reason why a much longer exposure to radiant heat might not have a more distributed, but essentially similar effect. If you put one cake in an oven at 400 degrees for half an hour, and another in an oven at 100 degrees for two hours, which will get cooked more? Does something hitting you one hundred times at 1 m/s have the same effect as it hitting you once at 100m/s? In your entire lifetime, you have been exposed to billions of times the heat as you experience when that metal burned you. Which caused more damage?

[Ririon]

Polarized windscreens....

What do you do with your nice polarized sunglasses when driving at night, guys?

If they really are polarizing they pass through 50 % (theoretical maximum) of the light intensity to your eyes. THAT is why polarized windscreens are as stupid as wearing your sunglasses at night. You might look cool, but you tend to bump into stuff, which is generally considered uncool. A judgement call... :cool:

Ririon

[Art Vandelay]

What I read- and this quite possibly is not true- is that only the very top of the windshield is polarized. When the sun is high in the sky, this cuts down on glare. At night, you don't need to see much there anyway. The top is also often colored blue, whereas sunlight is more yellow.

[Soapy Sam]

Originally posted by Art Vandelay
No, a burn is an injury caused by heat. A sunburn is not caused by heat, and it isn't really an injury; the trigger is UV exposure, but the cause is the body's reaction to it.

Then sunburn is not a burn by that definition. OK



If you put one cake in an oven at 400 degrees for half an hour, and another in an oven at 100 degrees for two hours, which will get cooked more? Does something hitting you one hundred times at 1 m/s have the same effect as it hitting you once at 100m/s? In your entire lifetime, you have been exposed to billions of times the heat as you experience when that metal burned you. Which caused more damage?

Obviously there will be a minimum heat intensity which results in a burn. Likewise a level which merely dries the skin. I don't know where the line would be drawn describing dermal dehydration as a burn. I do know it gets darn hot inside some bus windows, especially if you fall asleep with your arm or face touching the glass. You are technically correct, I'm sure, but I still would not recommend the practice.
And the damage done by low intensity heat / sun / environmental exposure generally is cumulative. Either that or someone hit me with an ugly stick when I was asleep.

[Ririon]

Why polarized sunglasses are so great for driving:

Light reflected off horisontal reflecting surfaces (like wet asphalt, water and the top surfaces of cars), especially when the sun is low in the sky, is partially polarized. Having glasses with polarization 90 degrees away from this direction will block a lot of this (annoying) light.

This is why polarized glasses are great for driving, fishing and other activities outside in the sun where there are reflecting horisontal surfaces.

Without any coloring, a straight polarization filter will just look like grey glass to the naked eye, unless you are looking at something at least partially polarized through it.

I can't think of any reason to add a polarization filter the top of the windscreen, except for tall people who have forgotten their sunglasses. They can just stretch their necks slightly. That doesn't mean that car makers don't do it, of course... As far as I know, there are no significant polarized light sources up in the sky, with the possible exception of alien spacecraft. It would reduce that annoying UFO glare at night... :cool:

Ririon