Comment by maybewhenthesun

I have to figure that one out every time I do it ;-)

The resulting interface ior should be positive if you go from a less dense medium into a denser medium, so I guess the material you're going to goes on top.

(which matches what happens from air-> glass. ior air is more or less one, mior glass = 1.5 so from air to glass -> ior 1.5)