Light versus dark is a recurring theme in Macbeth. There is a lot of light imagery in dialogues - specifically in the "tomorrow tomorrow" soliloquy. When Macbeth says, "Out, out brief candle," after learning of his wife's death, he reminds the audience of the strange act of his wife - always having a candle by her side. Since Lady Macbeth is dead, the candle that was always beside her is dead (blown out) too.
