Act 2.2-10 in Shakespeare's Romeo and Juliet uses metaphor and personification to describe Juliet, via Romeo's perceptions, and to thereby characterize Romeo. The figurative language creates the images of light and dark.
Juliet is metaphorically the sun, and an image of light is created:
But soft, what light through yonder window breaks?
It is the east and Juliet is the sun!
The sun, the light, is personified and set in opposition against the moon, the dark:
Arise fair sun and kill the envious moon
Who is already sick and pale with grief
That thou her maid art far more fair than she.
The sun continues to represent Juliet, here.
But Juliet should not be maid to the moon, since the moon is envious of her (continued personification) and the moon's outfit is sickly green (imagery), the kind of outfit worn only by court jesters, or clowns (imagery):
Be not her maid since she is envious,
Her vestal livery is but sick and green
And none but fools do wear it. Cast it off.
It is my lady, O it is my love!
Juliet is light, and she is contrasted with the moon (dark), and through his perceptions, Romeo is revealed to be totally and completely infatuated with Juliet.
To me, there are a couple of major points about this imagery.
First, it is warm, bright imagery that dispels the dark and cold. This is what is happening to Romeo's life as he "finds" Juliet. He is becoming happy and that is a bright and warm thing.
Second, there is the contrast between the sun and the moon. The sun (Juliet) is warm and loving. The moon is cold and does not give the same feeling as the sun. This refers to how Rosaline made Romeo feel in his previous "love." (He uses moon imagery when talking about her early in the play.)