I was concerned about the bright bottom and thought maybe there was a light tunnel effect. I looked through my stuff and found a dented cube, and its bottom seemed to show total internal reflection:
The light tunnel effect might be happening and there is a little glow under the cube, but you cant see it thought the cube. Elevating it a little does show that:
This is for debugging so the noise was just because I didn't run to convergence. That does look like total internal reflection on the bottom of the cube, but the back wall is similar to the floor. Adding a sphere makes it more obvious:
Is this right? Probably.