If glass blocks UV, how does it get out of the UV tube?

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Noel Eastwood

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Noel Eastwood  asked the Naked Scientists:

Hi Chris and crew,
 
How does my glass UV reptile light work if the ultra violet light can not go through the glass tube itself?

Thanking you in advance
 
Noel Eastwood
Menzies Creek, Melbourne, Australia


What do you think?

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Offline LeeE

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If glass blocks UV, how does it get out of the UV tube?
« Reply #1 on: 11/05/2008 13:23:21 »
From a little bit of digging around it looks like it's the UVB output that's the important factor in the UV lamps used for reptile vivaria.  The glass used in the lamps is fused "quartz" glass and not the more common soda-lime glass, the difference being that quartz glass is nearly pure silica whereas soda-lime glass has sodium carbonate, calcium oxide, magnesium oxide and aluminium oxide added to it and is only between 70-74% silica.  It would seem that it's the added impurities that block the UVB in ordinary soda-lime glass.

http://www.uvguide.co.uk/fluorescenttubeintro.htm
http://en.wikipedia.org/wiki/Glass#Glass_ingredients
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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Offline chris

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If glass blocks UV, how does it get out of the UV tube?
« Reply #2 on: 11/05/2008 22:47:41 »
That's a really nice answer; thanks Lee.

Presumably most fluorescent lights used soda lime glass to ensure that UV does NOT escape from the tube?

Chris
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Offline techmind

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If glass blocks UV, how does it get out of the UV tube?
« Reply #3 on: 12/05/2008 11:58:40 »
Glass attenutates UV to varying extents, least at the only-just UV end (UV-A - c.350nm) and essentially blocks UV-C (c. 250nm) outright.

"Blacklight" tubes used in clubs and bars for making clothes and drinks fluoresce primarily emit 365nm UV (a mercury emission line) and the tubes are made of glass. For this application a filter is coated on the outside of the glass to block the bluey-white visible light which is also generated by the mercury discharge in the tube.

"Germicidal" lamps which rely on the 254nm mercury emission line are made of quartz glass.


I can't find it now, but I made a post a few months ago (possibly about sun tanning/burning through glass) which linked to a transmission-spectrum for soda-glass.
"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"