Just as a pure curiosity question, that I have not been able to google successfully, and for clarity I will word the same question a couple of different ways- What ELEMENT is the most opaque? Meaning- How thin can the most opaque element be before it will allow light to penetrate it at any level? When I was in grade school, early 1960's our science teacher said that it was GOLD and I have wondered about it all these years. Personally I'd be willing to bet that gold is NOT CORRECT, or is it? The reason I think gold is not correct is that gold is used in a lot of higher end welding lenses to darken the lens against the electrical arc and is offered in various "tints" to suit the individual. On the other hand, he may in fact have been correct and that is exactly why gold is used. Thin can defined in terms of layers of molecules (100 molecules thick, etc) or an actual thickness like 1/10th of a hair thick or .00003" Expressing it in some (off-the-wall) scientific term wouldn't do a lot of good if I have no frame of reference (yeah, I'm not the smartest brick in the wall). Also I assume that the power of the light source would enter into it, so to keep the playing field level on that, let's say direct sunlight as the source. Hmmm... brings up another question- How many candlepower is the sun? To narrow the parameters a bit more, the brightness of the sunlight at sea level or here in SLC at approx 4,000 ft ASL. Well, might as well ask this also- what element is the most transparent? I'll assume it is hydrogen unless corrected, but is it fair to compare a gas to a solid? So I will amend the question to what SOLID element at room temperature (70 degree F.) is the most transparent? Thanks! -Barrett www.FallenStarHunters.com www.BarrettsCustomLeather.com