The sun gets a fair number of mentions in The Flies, and none of them seems congruent with the rest. When the play opens, the sun is a miserable, scorching torch in the sky:
I can't think how you bear it – this emptiness, the shimmering air, that fierce sun overhead. What's deadlier than the sun? (1.1.2)
Shortly after, it's a foreboding backdrop in a murder scene:
The people of Argos saw their faces dyed red by the sunset, and they saw them leaning over the battlements, gazing for a long while seawards. And the people thought, "There's evil brewing." But they kept silence. (1.1.36)
And then, all of a sudden, it's a happy addendum to a peaceful scene in paradise:
In Greece there are cities where men live happily. White, contented cities, basking like lizards in the sun. At this very moment, under this same sky, children are playing in the streets of Corinth. […] The sun is shining. Everywhere down in the plains men are looking up and saying: "It's a fine day," and they're happy. (2.1.77-81)
Out there the sun is rising, lighting up the roads. Soon we shall leave this place, we shall walk those sunlit roads, and these hags of darkness will lose their power. The sunbeams will cut through them like swords. (3.1.52)
What's going on here? Remember in Orestes's "Character Analysis" when we talk about "facticity"? Facticity refers to events in the past that we cannot change. Orestes can't change the fact that his father was murdered and he was exiled as an infant. But what he can change is the way he interprets that facticity and the value he assigns it. Things in nature – like the sun – work exactly the same way. The sun itself has no inherent meaning. It is neither scorching hot nor pleasantly bright nor foreboding nor uplifting until we decide that it is. What the sun represents depends on the human who is viewing it and the value he chooses to assign it. That's why the sun can mean so many different things in the span of a single day in The Flies.
Now let's look at the final passages from of The Flies. What has the sun come to represent to Orestes at this point? How does he choose to interpret it in Act III?
[The Tutor half opens both leaves of the door and takes cover behind one of them. The crowd surges forward, thrusting the doors wide open; then stops, bewildered, on the threshold. The stage is flooded with bright light. Shouts rise from the crowd: "Away with him! Kill him! Stone him! Tear him to pieces!"]
[Who has not heard them] The sun! (3.1.175-6)
[He strides out into the light. Shrieking, the furies fling themselves after him.] (3.1.180)
The play's final line (the stage directions above) is particularly important when you remember how the furies threatened Orestes and his sister earlier in the act. "You shall never see the sun again, Electra," they warned. "We shall mass between you and the sun like a swarm of locusts; you will carry darkness round your head wherever you go" (3.1.54). While both Orestes and Electra fled the temple – into the light – only Orestes realizes that he's walking in the sun (and chooses to interpret this positively, as opposed to the Tutor's "scorching sun" from Act I). Electra, though the furies have physically left her alone, will carry their darkness with her in the form of guilt and remorse forever.