One place in which images of light and darkness are merged is in the famous balcony scene. In particular, images of the sun are used as images of light, while images of nighttime, like the moon, are used as images of darkness. Shakespeare uses the images of light and darkness for several reasons. One reason is to symbolize the sexuality that is a dominant theme.
Romeo's feelings of sexual attraction for Juliet are especially expressed using light and dark images in his opening soliloquy in Act 2, Scene 2. He compares Juliet to the sun in order to capture her beauty, as we see in his lines, "What light through yonder window breaks? / It is the East, and Juliet is the sun!" (2-3). Since the sun is the brightest object in the sky, comparing her to the sun describes her beauty as being glorious and bright, just like the sun.
More importantly, he also uses nighttime imagery to express his sexual desires for Juliet. He uses an extended metaphor likening Juliet to the moon's handmaiden, telling her in his mind to kill the moon and cease being her handmaiden. The moon literally refers to the Roman goddess Diana, goddess of the moon and childbirth who is especially known for her vow of chastity. Hence, when he tells her in his mind to cease being the moon's handmaiden, he is really telling her to give up her chastity, which we especially see in his lines, "[the moon's] vestal livery is but sick and green, And none but fools do wear it. Cast it off" (8-9). Since vestal livery can be translated as virginal uniform or clothing, Romeo is metaphorically in his mind telling Juliet to cast off her clothing. The nighttime imagery is especially significant because it is typically at nighttime when sexual activity takes place. Hence these two images of light and darkness combine to paint a provocative picture of Shakespeare's ongoing sexual theme.