Romeo compares Juliet to light throughout the story of Romeo and Juliet. When Romeo sees Juliet for the first time, he exclaims that she teaches "the torches to burn bright." She is also "the sun" who can "kill the envious moon." He later says that her eyes are like "two of the fairest stars in all the heaven." She "hangs upon the cheek of night as a rich jewel in an Ethiop's ear." If Juliet dies, she wants Romeo cut "in little stars and he will make the face of heaven so fine that all the world will be in love with night and pay no worship to the garish sun."
Time:
Romeo and Juliet's relationship lasts for a very short amount of time. Time is also connected to the theme of light and dark. Shakespeare uses references to the night and day, the stars, the moon, and the sun. He sometimes has characters refer to days of the week and hours of the day to help readers understand that time has passed in the story. Time plays an important role when the letter explaining Juliet's "death" is not received on time and because of that, Romeo, and eventually Juliet kill themselves.
Poison:
Poison is the reason for Romeo and Juliet's deaths. (Technically Juliet stabbed herself, but that never would have happened if not for the sleeping potion.) The poison symbolizes the Capulet and Montague feud. Romeo and Juliet's love is "poisoned" by the feud between the two families.
In Act 2 Scene 2, Romeo says, "Call me but love and I'll be new baptized."
Baptism is a symbol of rebirth and welcomes new Christians into the community of faith. By saying that Juliet's love will "baptize" him, Romeo is saying that Juliet's love will make him reborn.