Frozen outdoor

[Andrea B.]

[[[Off Topic: John, I had intended but didn't write "another". So I went back and edited that and added an Edit note. No slight intended! I did mention that "we have been trying to make that known" because I know your work showed it. It is getting difficult to keep up with all the info now on UVP. I try to cross reference as much as I can when commenting. ]]]

[enricosavazzi]

Thanks for interesting information. The last sentence made me associate and wonder: Ten years back I bought one of those Steripens for an Africa trip. (As I recall it is based on a mercury tube.) When active it will in addition to UV emit a pale blue visible light. A sensor switch only allows it to be on with the tube submerged. The manual states that the emitted UV will not pass from the water to air. I have some trouble believing this really true? Should I wear UV protection glasses when using it?

I believe the amount of UV emitted by this type of device is tiny to start with, or their sale to the public would be restricted or accompanied by detailed warnings. Most of the UVC is likely absorbed within the first few cm of water, and the rest by the water container (bottle or drinking glass).

 

However, there is no physical reason why UV should not be able to pass the water-to-air interface. Total internal reflection is controlled by Snell's law, the formulation of which contains the refraction index of the two media (which is wavelength-dependent, so the critical angle for UV is not exactly the same as for VIS, but this is just a detail). Therefore, if you can see the UV tube through the glass of the container or the surface of the water, in principle UV will be able to travel along the reversed path (give or take some small difference) to your eyes, unless absorbed by some medium along its way.

 

Another interesting consequence (more from a theoretical than a practical point of view) is that if you hold the glass/bottle in your hand, skin oils and moisture will increase the critical angle (because the glass-to-air interface changes to a glass-to-oil or glass-to-water interface), so more of the VIS and UV will be able to leave the glass. This is the same phenomenon exploited in immersion objectives and condensers for high-NA optical microscopes.

[otoien]

Thanks for the Steripen comment. Attenuation in the medium sounds like a reasonable explanation. One of the conclusions in the article that Ulf provided link to was that stirring (for close contact with microorganisms/viruses) was very important for the effectiveness - without stirring effectiveness dropped from 99.99% to about 95%.