How to play
A laser on the board sends out a beam of light. Your job is to light every target by bending that beam with the pieces in the tray below the board. Put a piece on an empty square and turn it until the beam goes where you want. The beam is redrawn the moment you place, move or turn a piece, and while you drag a piece it already shows where the light would go. When every target glows, the level is solved.
There are 30 levels in three stages. The first nine use only mirrors. Levels 10 to 20 add beam splitters and boards with two lasers, and levels 21 to 30 add colour filters and coloured targets. Some levels also have fixed pieces on a bolted plate: the beam uses them, but you cannot move or turn them. Every level has exactly one solution, and it uses every piece in the tray.
Controls
- Mouse: drag a piece from the tray to an empty square, or click a piece in the tray and then a square. Click a placed piece to turn it, drag it to move it, or drag it off the board to take it back. A right-click also takes it back, and R turns the piece under the pointer.
- Touch: tap a piece in the tray, then an empty square. Tap a placed piece to turn it, drag it to move it, or drag it onto the tray to take it back. The page does not scroll while your finger is on the board.
- Keyboard: the arrow keys move a cursor. Enter or Space places the selected piece on an empty square or turns the piece under the cursor, Delete takes it back and the number keys choose a piece in the tray. Z undoes, Y redoes, H gives a hint, L opens the level list and F switches full screen.
- Buttons: Levels, Undo, Redo, Reset (all pieces go back to the tray, and Undo brings them back), Hint and Full screen.
Hint fixes one thing at a time: a piece that does not belong goes back to the tray, a piece facing the wrong way is turned, or the next piece of the solution is placed for you.
How the light behaves
Mirrors. A flat mirror follows the law of reflection: the angle of reflection equals the angle of incidence, both measured from the normal, the line at right angles to the mirror. A beam that arrives at an angle of incidence θi is turned by 180° − 2θi.
Every mirror in the game stands at 45° to the grid, so the beam always turns by exactly 90°. A “/” mirror sends light that travels right upward, a “\” mirror sends it downward, and both sides reflect.
Beam splitters. A beam splitter is a partly reflecting mirror, such as a glass cube with a thin coating. Part of the light passes straight through and the rest is reflected, so one beam becomes two. A real 50:50 splitter gives each beam half of the power; in the game both count at full strength.
Colour filters. White light is a mixture of many wavelengths. An absorbing filter lets one band of colours through and absorbs the rest, so a red filter passes red light and stops most green and blue. White light therefore turns red behind a red filter, a red beam passes a second red filter, and a red beam that reaches a green filter is stopped. A coloured target lights up only for its own colour. Most real lasers give out a single colour, but the lasers in this game send white light so that the filters have something to filter.
How the levels were made. A computer search follows the beam from each laser and, at every empty square the beam reaches, tries leaving it empty or putting any remaining piece there in each direction. A level was kept only when exactly one arrangement lights all targets, and a second, separately written solver confirmed this for all 30 levels.
Tips and strategy
- Work backwards from a target. Light can only arrive along its row or its column, so the last piece before it has to sit somewhere on one of those lines.
- A target stops the beam, so one beam lights one target. You need at least as many beams as targets, and every splitter adds one beam.
- When two branches need different colours, the filters go after the splitter, one on each branch.
- A dashed red ring around a target means that a beam of the wrong colour reaches it.
- There is no clock, so try ideas freely and use Undo. Only moves and hints are counted.
Frequently asked questions
Does every level have a solution?
Yes, exactly one. Two separate solvers checked each level, and in every level the single winning arrangement uses all the pieces in the tray.
Do I have to use every piece?
Yes. No level can be solved with a piece left over, so a spare piece in the tray means something is still not right.
Why does the beam stop at a filter?
The beam already has a different colour. A green filter absorbs red light, just as a real coloured glass would.