Light and dark serve as traditional symbols within William Golding's Lord of the Flies.
Darkness is a natural representation of evil, which can be seen in the awful deeds that take place during the night or under the cover of darkness. The threat of the beast seemingly becomes more potent at night, as well as the wild behavior of Jack's hunting, bloodthirsty tribe. Like the jungle around them, the darkness is untamed and uncivilized, and its presence influences the boys, corrupting their hearts and unleashing the most unsavory components of their human nature.
Light, on the other hand, offers safety to the boys, most often in the form of fire. The fire is the place where they gather to convene meetings, to stay warm, and to sleep. It is also the means of their opportunity for rescue (via the signal fire) and, in fact, is the cause of their ultimate rescue when a naval ship spots the immense fire that Jack has set in the jungle to smoke Ralph out of his hiding place. The beach, which is arguably one of the lighter places on the island, is the site of both their arrival and rescue; it is the point at which the boys are most in touch with civilization.
One way he uses light and dark is that the jungle is dark. This underlies a sense of danger and the idea that in the jungle lurks evil. In contrast, the beach and especially our initial description of the island seen through the eyes of Ralph is full of light.
Also, the need for a fire is significant. Fire is a symbol of light and more importantly, the fire must be made, watched over and never allowed to go out. The boys want the fire to be seen so that they may be rescued. Additionally it adds warmth and light in the darkness. Fire could be read as a symbol of civilization and the fear that they might lose the fire could symbolize the fear of the wild.