Version 2.79 (official) on Win10
I created a lens using a mesh with hyperbolic math function (z = sqrt(x^2+y^2 +1) and boolean modifier using a box to cut of.
I used glass BDSF shader, copied the lens with Ctr-D and rotated one of the two 180° on x-axis In render view one of the two looks as expected, the other one is completely black. I can reproduce it with exchanging both (the one with convex side on top is black)
What could be the reason ? (see pictures 1 and 2 with different view angels)