Don't Blind a Cell
A bulb that would blind every remaining source of some unlit cell is impossible.
How it works
- 1The crossed cell in the middle is still dark, and the four walls around it cut both its row and its column down to three cells.
- 2Three of the five cells in that cross are already ruled out, so only two can still light it: the one directly to its right and the one directly below it.
- 3Those two and the dark cell make three corners of a little square — and a bulb in the fourth corner would shine along its row into one of them and up its column into the other.
- 4That would leave the middle cell dark forever, so the fourth corner is empty. Only Light Source is the same idea with one candidate; this is the version for two or more.
- Cells that establish the pattern
- Cell this technique rules out