More at http://mathworld.wolfram.com/IlluminationProblem.html http://mathworld.wolfram.com/IlluminationProblem.html. (If curved mirrors are allowed the problem

More at http://mathworld.wolfram.com/IlluminationProblem.html http://mathworld.wolfram.com/IlluminationProblem.html. (If curved mirrors are allowed the problem was solved by Roger Penrose and there are rooms that cannot be illuminated - see link.)