What effect is Shakespeare creating for the reader with these contrasting images in ACT III scene II in Romeo and Juliet?
In Act III, scene 2 of Romeo and Juliet, Juliet hears that Romeo has killed Tybalt and that Romeo is therefore banished from Verona. She uses a series of contrasts to explain that her feelings are the opposite of what is expected of her. In addition, the contrasts she uses mirror the contrast between the lovers' wedding day--which is supposed to be festive and merry--and the reality that Romeo has murdered Tybalt.
At the beginning of the scene, Juliet praises night in terms that most people use for admiring daytime. She says, for example, "Whiter than new snow on a raven's back. /Come, gentle night, come, loving, black-brow'd night." She is asking for Romeo to be sent to her as if he were snow on a raven's back, and she thinks of night as loving and gentle--terms most people associate with day. These contrasts imply that only in night can Juliet and Romeo be together, so in the confused and tortured world of Verona, she must use night as a way to conceal their love rather than celebrating a public love in the daytime.
Later in the scene, Juliet laments over Romeo's actions:
"O serpent heart, hid with a flowering face!/Did ever dragon keep so fair a cave?/Beautiful tyrant! fiend angelical! /Dove-feather'd raven! wolvish-ravening lamb!/ Despised substance of divinest show! /Just opposite to what thou justly seem'st, /A damned saint, an honourable villain!"
She engages in a series of contrasts, such as "beautiful tyrant" and "damned saint," to express how confused she is that her husband, who is supposed to be arriving to celebrate their wedding day, is instead accused of killing her kinsman and is banished. Again, the idea here is that the lovers' fate is the opposite of what is expected on their wedding day. Instead of ending in happiness, their union will end in bloodshed and sorrow.