The image below is the horizontal cross section of a room. The bulb shows the position of the single light source.

When the light is switched on, one wall (marked in brown) remains completely in shadow.

enter image description here

Today’s puzzle is as follows: 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.

Clarification: A point is in shadow if there is no direct line from the light source to it.

Extra clarification: The straight wall sections need to join, so the cross-section of the room is a polygon. You can’t have a free-standing wall.

This puzzle is from Alex Bellos’ puzzle column in The Guardian.


2 Answers 2


Something like this (6 walls)?

enter image description here The 3 tiny walls are entirely in shadow, the larger walls only partially.

I assume that the polygon is not allowed to self-intersect, as that would not be regarded as “a room”. Otherwise a solution of 5 exists.


Here is the solution that I initially came up with:

enter image description here

The walls of the room are blue and the light is at the red spot.


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