For thin slabs of glass the same shader produces a very different effect. The parallel front and back surfaces conspire to avoid total internal reflection.

But even the external reflection rises with angle of incidence. I have marked the rotation angle of each slab in degrees.

Note that I used a volume absorption here because now it matters. When we look in the end face of the slabs, the internal reflection counts go well over 100 and we have to take into account the long path that the light took through all that glass, from one end to the other. As well, I had to increase my bounces to 128 to get the ends to show like real life.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)