The arrow pointing into the grid tells you where the laser beam enters. Draw horizontal and vertical mirrors on the intersections of the grid, such that the laser leaves the grid as indicated by the other arrow. The numbers on the left and top tell you how often the laser beam passes through a field of that row/column. The numbers on the right and bottom tell you how many mirrors you have to put on the corresponding line. The laser beam crosses itself only at the marked places. It meets each mirror exactly once.
(Rules and example from 24hPC 2007 IB)
History of the puzzle
First appeared on 2003 Dutch Nationals. Puzzle was written by Delia Keetman (Netherlands).
Original name Lichtstraal means "light ray" in Dutch.
Appearances in the WPCs
- WPC 2019/World Cup Playoffs by Jürgen Blume-Nienhaus
- WPC 2018/Round 13 (Coded) by Jiří Hrdina
- WPC 2013/Part 9