As you point out later in the thread the light can never touch any given sphere, since no matter which one you pick there will always be another sphere in front of it to block the light. At the same time the light beam must eventually hit something because the centre sphere is in its way. So your light beam must both eventually hit a sphere and never hit a sphere so your system is contradictory and thus ill defined. You could make the question answerable by instead asking for the limit of the light beam as number of steps of packing done goes to infinity in which case the light reflects back at 180°, since it does that in every step of the packing. Alternately you could ask what happens to the light beam if it is reflected of a shape which is the limit of the packing you described, in which case it will split in three since the shape produced is a cube (since it will have no empty spaces). (Edit:no it doesn't the answer to this question is again undefined via the argument in the first paragraph, since the matter it bounced of of had to belong to some sphere)

The answer "cubes with no empty space are filled cubes" was perfectly decent, as was the verbal argument about limits that Viliam provided. But in case those weren't satisfying, I've written a more explicit version of the proof* that the ray travels zero distance between reaching the cube and reaching a sphere. It utilizes the solution to the geometric sum, which is proved using limits: https://en.wikipedia.org/wiki/Geometric_series The proof (derivation?) is here: http://imgur.com/vSfABHk I think you can just argue by symmetry that the ray must retro-reflect. *At least, it looks like a proof to a physicist. It may not meet the standards of a proof to a mathematician.