Why can glass absorb ultraviolet light?

Question

Please see the following photos. (I cannot post them...)

http://i1163.photobucket.com/albums/q554/startanewww/CIMG4545.jpg

http://i1163.photobucket.com/albums/q554/startanewww/CIMG4546.jpg

From the first photo, the book mentioned Don't cover the paper with glass because glass absorbs enough ultraviolet light to slow the damage process. (line 8)

My questions:

• Why can glass absorb ultraviolet light?

• Will papers with dark colours fade fastest? Why is that the case? (see the second last paragraph)

Answer 1

Glass absorbs light where there is something in the glass that can resonate at the frequency/energy of that light.

It depends on exactly how UV you mean, at very short wavelength UV you can interact directly with the outer electrons, but at more typical 200-350nm you are mostly being absorbed by the inter atomic bonds in the glass. Unfortunately a lot of this is influenced by trace heavy metals and impurities so it's difficult to predict (and control)

There is a Schott datasheet explaining this

Q2: Darker colours fade faster because they have more dye and the change is more visible - it's hard to see when white paper has faded!
Dyes work by absorbing certain colours of light, so a red dye will absorb blue and green light to only allow red to reach your eye. When it absorbs the green and blue photons their energy must go somewhere mostly to heat, that's why dark cars or clothes get hotter on a sunny day. But some of the photon's energy will break the chemical bonds in the dye and so gradually reduces it's effectiveness = fading.

Answer 2

This is not a trivial answer. For the range 200-400 nm, regular borosilicate or soda lime glass does absorb enough light in order to make it not applicable for UV-Vis spectroscopy in which case quartz must be used. The differences between quartz and regular glass is two fold:

1. Impurities - quarts or pure SiO2, is crystalline and should contain little or no other elements. Regular glass ie pyrex contains up to 12% boron, other glasses contain aluminum, sodium, calcium etc.
2. Crystal state- regular glass has a more amorphous or less regular crystal lattice compared to quartz, the presence of impurities can also cause other cause other crystal habitats to form.

Both of the above reasons can cause the difference, not sure which though.