Does Glass Always Block UV Rays?

We know auto glass blocks some/most/all(?) UV light because we cannot suntan in a car with the windows up. But, what about when the UV’s incident angle is normal to the surface of the glass? Does it still not pass through? Also, I have read X-rays will not pass through glass. Can we conclude this is this true of glass for all lightwaves equal or smaller than UV waves?

Extra Credit: Can one safely observe a solar eclipse from within their car? I was told, perhaps as a simple answer, that while the UV is blocked, the IR is too intense for the eye. Maybe, but we also know IR cannot pass through glass hence the reason heat builds up inside a car as white light is absorbed and IR is emitted back. But, while glass is transparent for white light, it is opaque for IR. At least, this is what I was taught. What’s your understanding? - Jinx

NO
Glass will not block enough visible light or infrared to allow you to safely look at the sun. Smoked glass , one of the solar filters recommended by old books, isn’t really safe because the soot on the glass is too likely to come off when the glass is handled.

I would personally never try to observe a solar eclipse unless I were using something designed explicitly for that purpose, or were looking at an image of the sun projected through a telescope or pinhole camera.

It’s not safe to drive while looking through something that’s safe for solar viewing, either. ISTR a Darwin Award being given to someone who tried driving while wearing eclipse-filter glasses.

Jinx glass won’t block all UV, nor will it block all IR. Or to put it another way it ain’t completely opaque to either wavelength.

You won’t readily tan or burn under glass, and you certainly won’t under tinted auto glass. However that’s because the intensity is too low to cause tanning, not because all the UV s being blocked. The retina isn’t protected by any melanin or a layer of dead epidermal cell s as the skin is. When you try you look at the sun even through glass the UV is going directly into living cells. That’ definitely not a good thing since UV denatures proteins and nucleic acids. Bad mojo. Don’t try it.

Glass is even less effective at stopping IR. The reason heat builds up inside a car is not directly related to the inability of IR from the sun to pass through glass. If that were the cause then the inside of the car would be cooler since the IR light from the sun would be reflected back into the atmosphere and could never enter the car.

The reason a car heats up in the sun is because of the greenhouse effect. Light from the sun is largely visible and short wavelength IR light, both of which pass through glass quite readily. These wavelengths strike the car interior and the air within the car and energise the atoms within them. These energised atoms then re-emit long wavelength IR light as they cool down. This long wavelength IR can’t pass through glass and is trapped inside the car. As a result heat builds up steadily.

So it’s long-wavelength IR light that can’t pass through glass, not short wavelength. Long wavelength IR is lower energy light and is emitted from relatively cool sources such as animal bodies, warm stones and fires. The sun is definitely not a cool object and it emits copious amounts of short wavelength IR light that will go through glass without stopping. Looking at the sun through normal glass will result in a Ranier Wolfcastle moment:
“The goggles do nothink.”

The sun also emits a lot of long wavelength IR along with every other part of the spectrum but that’s irrelevant to this discussion.

Most glasses (and plastics) block light with wavelengths below 300 nm, or even slightly higher. That’s part of the UV range – light between 300 and 400 nm i considered UV as well, but not as dangerous (and you *can[/i get a bit o a tan with it).

However, there are glasses and plastics that trransmit UV – you just have to work at it.

Don’t try lookng at the sun or an eclipse through ordinary glass – there’s still enough bright visible light and long wavelength UV to cause roblems. I wouldn’t use “smoked glass” either – I wouldn’t trust it to be dark enough. Get extra-dark neutral density filters, or build a projection system.

People keep saying you can’t get a sunburn through auto glass, but I’m living proof that’s not true. I have to wear a shirt over my left arm when I drive, especially in the summer, or I’ll have a burn within 20 minutes or so. If I’m driving long distances in the summer, I have to cover up when the sun comes through the windshield as well. Now, I’m much more sun-sensitive than the average person, but if I’m burning, that means there are enough UV rays coming through to have some effect on anyone.

Glass doesn’t block X-rays either. It’s fairly effective at blocking low energy X-rays (order of 1 keV), but not very opaque at higher energies. Medical and airport security X-ray machines emit X-rays of around 60 keV. At this energy, a half-inch thick glass will only abosorb about 50% of the X-rays. Even less at higher energies.

(Actually that figure is for pure SiO2. Some glass have lots of impurities that change X-ray absorption. If it contains a high percentage of lead, for example, it can be an effective X-ray shield.)

So if you’re worried about X-rays, you should do all your looking through leaded-crystal stemware.

Technically, the retina is protected by the lens, which blocks wavelengths below about 380nm. That’s why patients who have their eyes’ lenses replaced with glass ones can see into the ultraviolet. Also, the fovea centralis is further protected by the macula lutea. What does the damage in the case of looking at the Sun is the heating effect from the sheer intensity of the visible and near infrared (out to 1500 nm I believe) radiation being focused on a narrow patch of tissue.

For the record, it has very little to do with the angle of incidence and more with the absorption spectrum of the glass. Most types of glass are highly absorptive in the UV region (though it is still possible to get sunburned, as another poster mentioned).

As Blake said, near IR passes through glass just fine (in fact that’s where the absorption is smallest), but the absorption starts increasing in the farther IR regions. In fact, it’s rather good that the absorption minima occurs in the near-IR, since everything you’re reading is being transmitted to you over single mode fibers made of fused silica glass at wavelengths around 1550nm, nestled quite comfortably in the near-IR.

Fair point, althiugh it has to be stressed that this is not a total blockage. UV damage to the retina occurs in people with perfectly normal eyes. Don’t look at the sun and always wear sunglasses while outside.

Also remember that the eye’s lens itself can be damaged by UV, causing cataracts.

Cecil addresses this question in part right here.

For safe eclipse viewing, specialized solar filters are ESSENTIAL. Sunglasses, film etc. might appear dark to visible light but will still transmit IR and UV.

http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/safety2.html
http://www.mreclipse.com/Totality/TotalityCh11.html

You can get sunburnt in a car but as you said, it takes long distances before you notice an effect. I guess it’s just like putting on a sunblock in a convertible !

but for your left-arm, you sure you aren’t driving with the side window down?