The basic setup of this issue is as follows: I have one floor, and one half-circular wall/roof to make a tunnel of sorts. The half-circle mesh is assigned a glass material and uses flat shading with about 50 vertices. There are several material-based light sources outside this tunnel.
When rendering, the light penetrates the glass material just fine, provided that flat shading is used. If I use smooth shading on the glass, the amount of light that makes it through is almost nothing. I know that some light still comes through, because I see a difference between a smooth-shaded render with and without the light source available.
While I'm fine with flat shading and I don't have a real problem with it, I'm more curious as to why this happens in the first place
EDIT: I've also noticed that smooth shaded objects don't reflect light as well as with flat shading, regardless of material transparency.