that's odd, I remember vaguely doing an expereiment in a micro lab where we held plastic petri dishes of bacteria under a UV light with and without the plastic covers on the dishes. The take home points of the experiment were that the bugs were sensitive to UV and that UV did not penetrate too deeply, i.e., through the cover of the dish. As for the idea that if glass blocked UV plants would die in greenhouses seems counterintuitive. If UV light is known to cause eventual DNA mutations... why would plants miss it? I believe plants absorb all visible light other than green -which is reflected back for us to see pretty leaves with. Most likely some UV penetrates through glass and plastic and that is why people put additional protection on glasses and windshields.

CDT

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2002suzukiGSF600S
1985 190e 2.3