[pasta]

My question was: "Why does it block UVB?".

Found the answer on Quora: https://www.quora.com/Why-does-glass-block-UV

"With a band gap of 4eV, glass can't absorb any photons with less energy than UVB light; namely, it is transparent to UVA, visible light, infared, etc; but the higher energy photons can and are highly likely to be absorbed."

So it seems hard to create glass that doesn't block UVB.

[kough]

Feynman knew this and claims to be the only person to have watched the Trinity test with naked eyes, rather than through welding goggles:

> In Surely You’re Joking, Mr. Feynman, American physicist Richard Feynman speculates that he may have been the only person who watched the Trinity Test relatively directly, using a windshield to exclude ultraviolet light. Everyone else, he claims, was looking through something akin to welding goggles.

https://www.sindark.com/2011/02/22/feynman-and-the-trinity-t...

[Bartweiss]

I've never really understood this story. Feynman wasn't wrong, but UV-A and even sufficiently-bright visible light also cause eye damage.

Did he know the main visual hazard from the bomb was UV-B? Did he just get lucky?

[gist]

> Feynman knew this and claims to be

I don't get this type of thinking. Unless he thought it would be worse to wear the googles what is to be gained by doing something like wearing goggles (in that situation) just in case you were wrong? Why not reduce the chance of harm as much as you can?

[BurningFrog]

The window didn't block a lot of other high power radiation, and he died from cancer 35 years later, though the connection is not scientifically certain.

[spuz]

There's a great Applied Science video on the various light absorption properties for various materials here: https://www.youtube.com/watch?v=vwsHRrDYu5o

[harperlee]

So in case anyone just panicked thinking “Wait, so do my car and office windows block cancer producing UV or not??”, this is what I found on cancercouncil.com.au (first google result):

- UVA penetrates deeply into the skin (the dermis) causing genetic damage to cells, photo-ageing (wrinkling, blotchiness etc) and immune-suppression.

- UVB penetrates into the epidermis (top layer of the skin) causing damage to the cells. UVB is responsible for sunburn – a significant risk factor for skin cancer, especially melanoma.

Which contrary to what I knew, links melanoma to sunburn, not DNA damage.

[JohnJamesRambo]

A good example of what UVA does would be the famous trucker with sun damage to one side of his face-

https://www.huffingtonpost.ca/2012/06/06/bill-mcelligott-sun...

[astazangasta]

Melanoma is definitely due to DNA damage; somatic mutation is the principal cause of all cancer. In this case the cause is either direct damage or damage by free radical byproducts created by UV. UV causes a kind of mutation in DNA called a pyrimidine dimer, where two adjacent bases mutate at once. By far the most common mutation in melanoma (reponsible for 50% of cases) is a CC to TT mutation at position 600 of the gene BRAF.

[logfromblammo]

The UV damages the DNA. That step is usually required to produce melanoma cancer. The sunburn causes the deeper cells with damaged DNA to multiply in order to replace the damaged cells.

A single quiescent skin cell with precancerous DNA can be cleaned up by the immune system. A precancerous cell that has multiplied itself to cover a patch of sunburn, activating some of the genes for rapid growth, is much harder to clean up.

[meschi]

Well, when you get sunburn i.e. high UVB exposure, you get high UVA exposure, too. My understanding is, that UVA radiation causes cancer.

But I would like to see some clarification on this topic, too.

[taneq]

> So in case anyone just panicked thinking “Wait, so do my car and office windows block cancer producing UV or not??”

I just figured automotive glass doesn't block UV (or at least all of it) since window tinting places always advertise UV blocking as a feature of their films. Cynically I know it could be just empty marketing, but it didn't seem like it.

[Nomentatus]

Of course, ordinary light, while still less energetic, penetrates still more deeply into skin, and it, too causes DNA damage. In situ DNA is damaged by light absorbed by less transparent molecules it is near.

[garmaine]

Why are you assuming there isn’t DNA damage when sunburned?

[Etheryte]

Quartz glass is transparent to the UV range, so it's not so much that we don't have the technology, it's just way more expensive than regular glass.

[masklinn]

Sapphire's also transparent down to far ultraviolet (150nm). Not sure about ALON.

[foobar1962]

Nikon released a lens in 1984 that offered transmission and correction from UV to IR -- from ultraviolet, through visible light, to infrared (near IR, not heat).

http://www.company7.com/nikon/lens/0105f4.5uv.html

This product was originally announced in 1984 as the Nikon 105mm f/4.5 UV-Micro-Nikkor, and from September 1985 it was marketed as the Nikon UV-Nikkor, then the lens sold then for $2,200.00 USD, then about half the cost of a full sized car.

[rjf72]

Mildly amusing related aside from this. Because of this, glass is completely opaque in the UV spectrum. Looking at the world in UV [1] is quite interesting! Such a reminder that how and what we experience of the world is so largely a product of our physiological composition. What is perfectly transparent to us would be a great hiding place from the perspective of something that only saw in UV.

[1] - https://www.youtube.com/watch?v=V9K6gjR07Po

[hopler]

Other comments say glass does not block UVA

[habosa]

My grandfather, who was an ophthalmologist with a passion for inventing, has shown me schematics for this kind of glass. His idea was that you could put it in places that are sunny but not warm (like a mountain hotel) to allow for indoor sunbathing.

We always make fun of him for this idea as it's one of his strangest. I don't think he ever finished filing the patent.

[tomcam]

I mean this as politely as possible: why would you make fun of someone developing in an unusual idea?

[devy]

> So it seems hard to create glass that doesn't block UVB.

Physics aside, why would you want to do that in the first place? UVB is the chief cause of skin reddening and sunburn and plays a key role in the development of skin cancer and a contributory role in tanning and photoaging. [1]

Just for that little benefit of triggering Vitamin D synthesis is not worth the increased risk of skin cancer IMO. And the author lays out the alternative there too: "Those concerned about low vitamin D levels can get more of the vitamin through foods. "

[1]: https://www.skincancer.org/prevention/uva-and-uvb

[tlals]

There is an alternative view that regular sun exposure is beneficial, and that many of the health benefits linked to vitamin D are actually just using vitamin D as a proxy for sun exposure. If this is the case, it's plausible that allowing for more UVB exposure indoors would be a net benefit.

Previous HN discussion: https://news.ycombinator.com/item?id=18890475

[rhinoceraptor]

I don't doubt that sun exposure is a contributing factor to skin cancer, but it just does not make sense that skin cancer rates have shot up over the past 100 years, while at the same time people spend less and less time outside.

IMO, there's probably some other causal factor(s), and reducing sunlight exposure is not the solution.

Sunlight exposure is important for health, not just for Vitamin D (which others have pointed out, may just be a proxy for some other factor of sun exposure). It's important for regulating circadian rhythm, as well as preventing myopia in childhood.

[com2kid]

> Physics aside, why would you want to do that in the first place?

I worked on a product that had a UV sensor. It needed to be protected.

Sourcing glass that didn't block UVB, that could be used in a mass market product, at cost, integrated into a manufacturing line, was a bit of a challenge. The mechanical engineering team eventually got a hold of some. For awhile, there were weekly status updates of "got another manufacturing sample, spec sheet wasn't quite honest, it blocks some UVB."

[C1sc0cat]

Depends for some skin conditions its better to have timed exposure via UVB therapy - I have had this in the past.

[Florin_Andrei]

The answer to all such questions is - something related to energy levels of electrons in that material. If there's a resonance somewhere, photons at that energy get absorbed.

[amelius]

> So it seems hard to create glass that doesn't block UVB.

Ok, so what is the glass in tanning-booths made of? Or the glass of UVB fluorescent lamps?

[duskwuff]

Quartz.

[lightedman]

Soda-lime glass transmits pretty much all UV ranges and is readily manufactured (it is used often in germicidal lamps.)

[vanderZwan]

You can coat it - sunglasses are, for example. The coating can be invisible to the naked eye too, I think.

This is one of those situations where I think government intervention is needed. I bet the long-term benefits of coating glass like this are very real - both for society and individually (especially in professions that involve a lot of driving time). However, the short-term economic incentives work against it - there is probably a strong first-move disadvantage. Also, what is the economic benefit to a landlord to have UV-proofed glass for their tenants?

But if government were to implement a policy of requiring glass in cars and buildings to be coated like that? That levels the playing field. I doubt it is going to happen any time soon though.

But of we were ever going to to do that, I know for a fact that are also coatings with reflective layers (invisible to us) that tell birds that the glass is there, which would also save a lot of wildlife.

[Waterluvian]

I think you're inverting what parent is saying. Glass by nature seems to block UVB.

[georgyo]

How does coating it prevent the glass itself absorbing UVB?

[losvedir]

Ah, yes the classic "government intervention to attack a misunderstood problem with a solution that won't work".