But if your emitter emits in every direction (like a lightbulb) and you only catch the ones going in one direction in the sail, you'd propel yourself in that di

But if your emitter emits in every direction (like a lightbulb) and you only catch the ones going in one direction in the sail, you'd propel yourself in that direction. So having a directional lightsource fire into the sail wouldn't work, but having one that emits light in every direction on one side of the sail would.