Like Steve, playing around with it, there doesn’t seem to be a way to stop refraction even at 1.0. We tend to think of 1 as no refraction but it’s the refraction of air which does have refraction. You can get what you want (at least in this test) in the Corona renderer since it has a switch to turn off refraction (for glass panes etc where refraction is not needed would just slow things down to no effect) but nothing like that that I know of in the standard renderers.