Always a hard question: do I have a bug?

In testing some new code involving box-intersection I prepared a Cornell Box with a glass block, and first question is "is this right?".    As usual, I am not sure.    Here's the picture (done with a bajillion samples so I wont get fooled by outliers):

It's smooth anyway.   The glass is plausible to my eye.   The strangest thing is how bright the bottom of the glass block is.   Is it right?    At first I figured bug.   But maybe that prism operates as a light tunnel (like fiber optics) so the bottom is the same color as a diffuse square on the prism top would be.   So now I will test that hypothesis somehow (google image search?   find a glass block?) and if that phenomenon is right and of about the right magnitude, I'll declare victory.