I have been asked a question by someone not happy with the "Just because it does!" answer. Apparently when I learned this fact I was happy with that answer. Anyway, the question is, why does welding spatter bounce off plastic lenses but burn glass lenses.
My guess is that the greater thermal conductivity of glass causes an instant transfer of heat from the spatter to the glass, causing burning of the surface by a now solidified spatter, whereas the lower conductivity of plastic leaves the spatter as hot liquid, thereby bouncing off the surface.
Another possible answer is that plastic is more hydrophobic than glass, repelling the liquid spatter.
The problem I can see with either of my theories is the time factor. That is, is there time for these effects to take place?
Darryl and Steve, given your intimate knowledge of CR-39 manufacturer, perhaps you've thought about this before. Incidentally, Darryl, I've just read you post about your move. I'm sure that Sola's loss will be some other astute company's or college's gain. It's a real shame that Missouri is not close to Sydney!