The imminent release of the first of the Tolkien trilogy on film reminds me that JRRT made extensive use of symbols involving light vs darkness all through LOTR. The gift of the lady Galadriel to Hamfast, the twilight into which Frodo falls when he wears the ring, the doors whose key can only be seen by starlight, the all-pervasive gloom of Mordor, etc. etc. Now that I come to think it over, "extensive" is not the word -- it's more than that. Indeed, the contrast/conflict between light and dark is almost a theme running under the whole work.