Why do windows block UVB, but not UVA rays?

96 views

I read that windows block majority of sun radiation, some even claimed that windows block almost 100% of UVB, but I found no reliable sources on windows blocking UVA. I guess it is due to the wavelength and UVA having longer wavelength means they penetrate deeper, but why is that so ?

Edit: I formulated my question poorly, instead I should’ve asked: what prevents us from creating windows that block UVA rays?

In: 2

4 Answers

Anonymous 0 Comments

Search for “soda lime glass transmission spectrum” and you get how typical window glass let through light depending on the frequency. The exact spectra will depend on the glass but all show that it go from 90% transparent to quite close to 0% transporter around 300 nm

[http://www.vidrasa.com/img/esp/trans_arglass.jpg](http://www.vidrasa.com/img/esp/trans_arglass.jpg)

UVA is defined as 315–400 nm so it will let a lot of light through for it and block practically all UVB,

The way question is because that is the color of soda lime glass. That is color in UV, visible, and IR light. You can see in the graphs that are starting to block IR light. It drops to semitransparent around 2500 nm and blocks all at 5000 nm.

An emerald is green because it lets green light through and absorbed another part of the visible spectra look at [https://www.geminterest.com/images/spectrabs/Vis-NIR/VNIR_EmeSyntHydroVertFoncRu.jpg](https://www.geminterest.com/images/spectrabs/Vis-NIR/VNIR_EmeSyntHydroVertFoncRu.jpg)the 500 nm peek and a lot trough up to 550 nm and that light we see as green.

When we usually talk about color we only care about it in the visible part of the spectrum but it is no different in UV and IR light. The difference is just you can see it but instruments can.

Another glass has other spectrums like borosilicate glass [https://www.continentaltrade.com.pl/files/MIKA/SZKLO/Szklo%20wodowskazowe%20i%20wzierne/Continental/Przepuszczalnosc%20szkla%20420_EN.png](https://www.continentaltrade.com.pl/files/MIKA/SZKLO/Szklo%20wodowskazowe%20i%20wzierne/Continental/Przepuszczalnosc%20szkla%20420_EN.png) that is common in cookware and uses in UV light bulb that is defined to emit UV light. It starts to block UV light at 200nm

So different glass that can look identically to you in visible light can behave quite different in UV and IR light

There is a complicated explanation for why that is the color of material by using how light interacts with atoms and molecules. It is out of the scope of this post and is a complicated explanation of why stuff has the color they have.

So the simple explanation is it lets through most UVA light because of the color of the commonly used soda glass in windows.

Anonymous 0 Comments

We make windows out of material that allows visible light to pass through, that is the whole point of a window, and UVA is the type of UV very close to visible light and so it makes it through. [Here is a diagram for refference](https://materion.com/-/media/images/business-units/advanced-materials-group/me/newsletter-images-2016_targeted-emails/uv-in-electromagnetic-spectrum_600.jpg).

Anonymous 0 Comments

UVB is the shorter and more energetic of the two types of UV radiation.
Since the wavelengths of UVB are smaller, they are more likely to be
absorbed by glass. UVA is the longer and less energetic of the two types
of UV radiation, so it is more likely to pass through glass without
being absorbed.

Anonymous 0 Comments

UVA and UVB are different wavelengths. It’s the same reason things have colors. Different materials absorb, reflect, or allow through different wavelengths of light. We see different colors in the visible light range, but the same principle still applies for nonvisible light.

It’s like how the old blue and red 3D movies worked. You have a blue lens and a red lens, and the movie is projected with blue light and red light, so only the blue light goes through the blue lens, and only the red light goes through the red lens. Glass is just a lens that let’s through visible light and UVA.