More at <a href="http:&#x2F;&#x2F;mathworld.wolfram.com&#x2F;IlluminationProblem.html" rel="nofollow">http:&#x2F;&#x2F;mathworld.wolfram.com&#x2F;IlluminationProblem.html</a>.<p>(If curved mirrors are allowed the problem was solved by Roger Penrose and there are rooms that cannot be illuminated - see link.)