In The Great Gatsby, the green light that appears across the water from Gatsby's mansion symbolizes Gatsby's hope and longing for his lost love, Daisy Buchanan. The green light represents Gatsby's dream of being reunited with Daisy and rekindling their romantic relationship.
As the novel progresses, the green light takes on a more complex meaning, coming to represent not only Gatsby's hope but also the corrupting power of materialism and the illusory nature of the American Dream. The green light, like Gatsby's dream, is ultimately revealed to be unattainable and fleeting.
In the final chapter of the novel, Nick Carraway reflects on the green light and notes that it has "vanished forever" from Gatsby's sight. This disappearance symbolizes the final collapse of Gatsby's dream and his ultimate failure to win back Daisy's love. The green light's disappearance also underscores the novel's larger theme of the impossibility of recapturing the past and the dangers of living in a world of illusions and false dreams.