Glass and Sunburn

richardm

Philosopher
richardm
Joined
Aug 6, 2001
Messages
9,248
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?
 
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.
 
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.
 
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.
 
Iconoclast said:

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

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.
 
Ladewig said:
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.
Click to expand...
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.
 
Iconoclast said:
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.
Click to expand...

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?
 
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.
 
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!
 
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.
:)
 
So... I hope nobody minds if I just skulk away now, I'm having a bit of a shocker today.
 
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.
 
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
 
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:
 
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.
 
Soapy Sam said:
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
Click to expand...
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...
 
Iconoclast said:
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...
Click to expand...
And whoever told that gets this:

:bs:
 
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.
 
Iconoclast said:
I was told quite specifically about this polarized area on the windscreen way back when I was in high school.
Click to expand...

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
 
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