OK, on the left we have a surface of the giving forth of the light. There isn't any other light falling on the scene, which is why you can't see the black cat in the background (that's a sort of joke). The big ball has the same simple grey material as the ground plane. The two smaller balls share the same simple mirror material (just reflection - nothing else) which is also applied to the flat mirror at the back. As you can see, the light coming from the bright surface is interupted in its path by objects - in other words it casts shadows. This is an optical property. They're nice soft shadows, since the light source is the diffuse sort that set me searching all those months back. I hope it also plainly evident that the light is being bounced back into the scene from the mirror at the back. Closer examination will reveal the little curvy caustic reflections given off by the small spheres.