Certainly for Romeo, the comparison to Juliet to light is effective in communicating his own feelings towards Juliet. For Romeo, Juliet represents light in several ways. Romeo seeks to find his way out of the melancholy he feels with Rosaline. This state of darkness is illuminated with Juliet. When Romeo pursues her, she serves as the "light" for him that guides him through the dark of night. As Romeo finds himself in an abyss regarding what to do, his affections for Juliet light his way. It is in her presence that he feels meaning is evident. She serves as this force of illumination in a condition where there is much in way of darkness. When she is near him, he feels that there is light and goodness present. When apart, he experiences darkness. In this, the comparison to light and Juliet is an effective one.
As his speech and presumably his feelings become more eloquent and more intense, Juliet's presence has served as a light for him. Romeo feels that his ascent into a world of illumination, of seeing consciousness clearly, is because Juliet is present within it. For this reason, the comparison to light is suitable in being able to describe Romeo's feelings for Juliet.