The excerpt with the green light symbolizes foreshadowing of longing and elusive dreams in The Great Gatsby.
The best example of foreshadowing in The Great Gatsby is the excerpt:
"...he stretched out his arms toward the dark water in a curious way, and, far as I was from him, I could have sworn he was trembling. Involuntarily I glanced seaward- and distinguished nothing except a single green light, minute and far away, that might have been the end of a dock. When I looked once more for Gatsby he had vanished, and I was alone again in the unquiet darkness." This excerpt foreshadows the themes of longing, hope, and elusive dreams that are central to the story.
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