One symbol used by F. Scott Fitzgerald in The Great Gatsby is the green light at the end of Daisy's dock. At the end of Chapter 1, narrator Nick Carraway observes Gatsby standing out on his dock and reaching out towards the water with a "trembling" hand, and all that Nick could see was a "single green light" out in the distance. Later, we learn the green light is always kept lit at the end of Daisy's dock. The green light is a very effective symbol of Gatsby's hopes and dreams that prove to be nothing more than illusions. The color green can symbolize rebirth, life, and vitality, as well as that which is associated with fertility and life, which is sexuality and even romantic love; since green is also the color of US money, the color green can also symbolize monetary gain or monetary desire. However, light is literally not something that is graspable; it can be seen but not held firmly in the hand. Hence, all in all, the green light serves to symbolize all that Gatsby desires--money, love from Daisy, sexual fulfillment shared with Daisy--but can never actually truly grasp.
Later, it becomes clear that the billboard over the valley of ashes advertising optometrist Doctor T. J. Eckleburg symbolizes God's judgement of a society that has crumbled in moral decay. The billboard is described as depicting eyes wearing a pair of eyeglasses. The interpretation of the eyes representing God's eyes becomes clear when, after Myrtle's death, Wilson looks at the billboard and comments, "God sees everything" (Ch. 8). Due to the disturbing description of the billboard and Wilson's comment, the billboard is another effective symbol used to underscore the theme of moral decay.