Guardian Puzzle Feb 1, 2016
The Guardian Puzzle of Feb 1, 2016 - https://www.theguardian.com/science/2016/feb/01/can-you-solve-it-the-shady-puzzle-that-will-keep-you-in-the-dark
The puzzles asks the reader to "design a room with straight walls in which there is a position for a single light source that leaves either part or all of every wall in shadow."
Bellow is my solution that guarantees that each wall of the 6 segment polygon AFBGCE is part in shadow. The source light is D and is in center of the ABC triangle. The second triangle EFG is slightly rotated inside the outer triangle and this allows the creation of three shadow areas each affecting two walls. Don't know if there is a simpler solution, but I don't there is.
Play with the Points of the Inner Triangle EFG to see how they affect the shadow areas.