TO add to what Ian said, here's a nice graph from Schott showing the IR transmittance of one of their IR absorbing glass:
Keep in mind the graph is in transmittance. If it was in absorbance, then you could more easily see that a sheet twice as thick had twice the absorbance.
Ray - Your questions #2 and #3 are more easily seen when using absorbance. That's why most all lab tests that use spectrophotometery use absorbance rather than transmittance, as the math can be done by using simple addition or subtraction.
Keep in mind the Beer-Lambert Law (aka Beer's Law):
It tells us that if the filter (or cuvette) thickness is doubled, then the absorbance is doubled.