Thanks, Dr. Dawg! I often wondered about that. And I didn't know that the useful parts of the spectrum were so narrow. Before, all I had seen were absorption spectra, which I thought had a fairly narrow "valley" around green wavelengths, not a big 200 nm gap.
That's good to know.
So it may not matter much to gardeners, but conventional window glass does absorb some, or a lot of, UV.
I forget whether it mostly absorbs short-wave or long-wave UV. We had sterile rooms in school, flooded with UV when not in use. When we worried about that UV passing through glass windows and frying our eyes, we were told "don't worry, glass absorbs UV". (Perhaps he meant that the more dangerous, shorter wavelength UV-B and UV-C are mostly absorbed.)
When optics in scientific instruments have to transmit all the UV, they have to use expensive quartz (fused silica, I think) instead of cheaper silicate glass. Of course, they also have to pump all the air out of the light path, to prevent absorption of "vacuum UV" and "extreme UV".
And yet, some UV, of some wavelengths, must pass through glass, because there are more expensive products available for blocking 98% or 99+% of UV, for example to protect million-dollar artwork from any fading.
So if some unusual plant species DID need UV (for example long-wave, low-energy UV-A), window glass probably would block a lot of it, but not ALL.
