Several examples of juxtaposition in "Romeo and Juliet" have to do with light contrasted with dark, as in Romeo's description of Juliet in Act I, Scene 5: "It seems she hangs upon the cheek of night/ Like a rich jewel in an Ethiope's ear." He goes on to say of her, "So shows a snowy dove trooping with crows/ As yonder lady o'er her fellows shows."
Juxtaposition is the state of being side by side or close together for the purpose of contrast or comparison. Shakespeare uses the imagery of light, day and the sun in close proximity to images dealing with darkness or night. In both of the above quotes, the light is made to seem even brighter by the darkness with which it is contrasted. In the first one, Romeo compares Juliet to a jewel, which, although it has plenty of sparkle on its own, is enhanced by its placement against dark skin. Comparing Juliet to other girls, Romeo likens her to a white dove among dark crows to demonstrate how she stands out among the rest.
Later in Act III, Scene 2, Juliet uses similar imagery when she is waiting on the night to arrive and Romeo to come. She states, "Come, Romeo, come, thou day in night/ For thou wilt lie upon the wings of night/ Whiter than new snow on a raven's back." To Juliet, the anticipation of night's arrival calls to mind the brightness in her life that Romeo brings. He is her "day."