In the field of optics, however, engy, its negligible when you're attempting to tell about the angular change. Poor word choice, but meh, I can see what he meant. Basically, no matter the density of the atmosphere, its negligible when compared to the kind of refraction Tom was talking about, which pretty much requires water, not air. The IOR of air is 1.0003. The IOR of vaccuum is 1. I would have said the IOR was mostly constant, not the density. Just cede the point soka, that you picked the wrong word, and can we get back on topic?