For many times in the play we get a reference to blindness/vision. Why is that? What is the intent or purpose of these many references?
The many instances to vision and blindness underscore the major irony of the play: although Tiresias is physically blind, he can "see" the truth of Oedipus' fate while Oedipus, though he has physical sight, cannot recognize the truth around him. The many instances of light/dark imagery help to reinforce this contrast. When Oedipus finally does come to understand ("see") that he has not thwarted his destiny but instead fulfilled it, he says, "Eyes, look your last!" before he gouges out his eyes and blinds himself. He no longer wishes to see evidence of what he's done by looking at his children or at the people of Thebes who now know the truth that Tiresias has known all along. Now Oedipus, like Tiresias, is physically blind, but he is able to "see" the truth.
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The repeated references to sight / seeing / vision and blindness are a literary device called a "leitmotif." By examining a leitmotif in its different contexts, we can arrive a theme of a literary work. Here it has to do with the metaphor of "sight" as knowledge, especially knowledge of the self. We "see" when we understand, just as we "see" when we turn on the lights; we don't see in the dark. In developing the leitmotif of "seeing" metaphorically, as knowledge, and literally, as physical vision, Sophocles provides an ironic relationship between the two. The blind Tiresias has knowledge, but the king who has vision lacks that until, at the end of the play when he finally discovers knowledge, at which time he takes away his physical vision.