Prism strategy: how to split the light and fill every gem

Prism hides nothing. Every beam is drawn, every gem shows what it wants, and the whole board relights the moment you place a piece. So solving one is never about guessing. The board arrives empty, you are handed a counted-out set of pieces, and the puzzle is deciding where the light should bend.

What you are given

Two kinds of piece, counted separately and shown under the board. Pick one before you tap, and picking is free: it costs no turn and nothing moves until you touch a square.

• A mirror turns a whole beam ninety degrees. Nothing carries on past it.
• A splitter turns half the beam and lets the rest carry straight on, so one beam becomes two. It is the shorter, thinner, glassier piece, and it is the only one that glows on both of its faces at once.

Tapping a square lays the piece you picked, tapping again turns it, and once more lifts it away. A spent piece leaves its empty slot in the rack, so you can always see what is left.

Get the white beam into the prism before anything else

Nothing on the board can be right until the white beam reaches the prism, because until it does there is no red, yellow or blue to route anywhere. The white beam is the brightest thing on the board and it comes from a single lamp, so it is easy to follow. Trace it and see where it needs to bend. And note that reaching the prism is not enough on its own: it has to arrive head on at the flat face, which is the next section.

The prism has a front, and only the front works

This is the rule that catches everyone first. A prism is a solid wedge of glass with a flat face and an apex, and it only does anything to light that arrives head on at the flat face. Light that strikes one of its slanted sides scatters and is lost, exactly as it would be in real glass. So the prism is not a passive splitter you can feed from any direction. It is a piece with a correct side, and finding the route that approaches it correctly is usually the spine of the whole puzzle.

You can see which face is the working one: it glows harder than the other two, and the apex points away from it. Red will leave through that apex.

Know which way each colour leaves

Light entering the flat face head on does not bend at all. All the bending happens on the way out, at the two slanted faces, and that is where the colours come apart.

Because the prism is fixed, those three exits are fixed too, which tells you a great deal before you place anything: you already know which direction each colour sets off in, and therefore which side its gem has to be reached from.

Colours mix like paint, not like light

This is the part that trips people who half remember school physics. Prism mixes the way a painter does, not the way a stage light does:

• An orange stone wants red and yellow together.
• A purple stone wants red and blue.
• A green stone wants yellow and blue.

A gem lights only when it is receiving exactly what it asked for and nothing else, so a stray beam of the wrong colour landing on a finished gem will put it out again.

Mixed gems are the ones that cannot be solved a piece at a time

A gem wanting a single colour needs one route to work, and you can see straight away whether you got it right. A gem wanting orange needs two separate beams to land on the same square, and until both are right it shows you nothing but a faint wash of the colour it is already getting. That faint wash is the most useful thing on the board: a stone that wants green and is already washed faintly yellow is telling you half the job is done and only the blue route is wrong.

The way through is to stop treating it as one problem. Solve the longer or more awkward beam on its own, ignoring the other entirely, then bring the second in to meet it.

Spend splitters where one colour has to reach two gems

If two gems both need red, you cannot do it with mirrors alone, because a mirror consumes the beam. Put a splitter where the red beam crosses the first gem's row or column, so it drops half the light into that gem and carries the rest onward. Save your mirrors for the last stop on each branch, where nothing needs to continue.

Count before you start. If a board gives you three mirrors and six splitters, it is telling you there are six places where light has to serve a gem and keep going, and only three dead ends.

When a beam sails past its gem

Look at the last piece that beam actually touched, not the one nearest the gem. A beam misses because of where it was last turned, and the piece sitting beside the gem is frequently not on its path at all. Follow the light backwards to the last place it changed direction, and start there.

Blocks eat light

The solid squares are opaque and any beam reaching one stops dead. They are not decoration: a block sitting on the obvious route is telling you the route is wrong, and often the whole shape of a board is set by which lines are closed off.

Ran out of pieces?

You have not lost. Lift one and put it somewhere better, or Restart and plan the whole route before you place anything. Restarting costs nothing, and picking a piece from the rack has never cost a turn, so thinking is always free.

Quick checklist

• White into the prism first, and head on at its flat face. Nothing else can be right before that.
• A slanted face scatters the light and loses it. The prism has one working side.
• Red carries straight out of the apex, yellow bends left, blue bends right.
• Red and yellow make orange, red and blue make purple, yellow and blue make green.
• A mirror ends a beam. A splitter keeps it going. Use splitters where a colour must serve more than one gem.
• For a mixed gem, finish the harder beam first, then bring the second one in.
• The faint wash on an unlit gem tells you which half is already right.
• If a beam misses, go back to the last piece it touched.

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