What is the significance of Tom saying "the world is lit by lightning now" before he tells Laura to blow out the candles?

Though THE GLASS MENAGERIE is ostensibly about Laura, the opening and closing speeches of the play tell us that this story is really about Tom, and his quest to make peace with his memories, and his guilt at having left his sister in the grips of Amanda, left her as his father left them long before. His journey is very much a portrait of an artist - as a poet (let's be frank, a playwright-in-training, knowing the autobiographical nature of the play), Tom is desperate to escape the confines of his bleak, unpromising home life and discover the romance and power of life. When he talks of the world being "lit by lightning," what he means is that he has made the plunge - he has left the quiet sadness of a world lit by his sister's candles and traded it from the unceasing excitement of a lightning storm. He asks Laura to blow out her candles - in effect, for her memory to stop haunting him - because he no longer sees the world by the dull light of those. Instead, his world is lit by the excitement of lightning. As much as it pains him to do it, he needs to leave her behind and embrace the decision he has made to live in the heart of lightning.