Light and dark imagery can symbolize many different things in Romeo and Juliet. Literary critic Clifford Leech argues that the contrast between light and dark imagery shows that, since their love is separated by their feuding families, their love is something unnatural, something that cannot fully exist in the world, but rather something that is doomed to failure as expressed by the dark imagery (Leech, "Rome and Juliet: Romeo and Juliet").
Light imagery is especially used to describe Juliet's beauty, showing us that Romeo sees her as more of a celestial being rather than a real person and that his love for her is otherworldly. Light imagery is especially used in the famous balcony scene when Romeo sees Juliet at her window. Romeo makes an analogy between Juliet and the sun, as we see in his lines, "But soft! What light through yonder window breaks? / It is the East, and Juliet is the sun!" (II.ii.2-3). Since the sun stands above the world, the image portrays Juliet, as well as Romeo's love for her, as otherworldly.
However, while their love is described with light imagery, their meetings always take place at night. One reason is that most amorous activity takes place at night. But another reason, as literary critic Leech suggests, is that the darkness they are in is literally crushing and killing their love, just like their families' feud. As Leech states, "Their love cannot--which is the mark of its doom--exist in the sun, its natural element," which Romeo shows in is paradoxical line, "More light and light; more dark and dark our woes," which he utters just as dawn is breaking the morning after their secret wedding night (Leech, "Rome and Juliet: Romeo and Juliet"; III.v.36).
Hence we see that the light and dark imagery can portray beauty, their love, and also the struggles they are enduring due to their families' hatred.