Light bulbs do not emit light. Really.
In reality, they suck up the dark.
Ever notice how black a burned-out light bulb looks? That means it's full of dark and needs to be replaced with an empty one. Further, when a bulb is nearly "full", it may flicker. If you tap it, and knock some of the dark out, it has room for more and thus will "light up" again!
good friday afternoon chuckle