The scene begins when Romeo speaks about the rising sun and compares Juliet to it and says that the moon is 'envious' of Juliet's beauty. Juliet is described as a 'bright angel' so light is being seen as something positive.
However, later in the scene darkness is described as 'night's cloak' which means that Romeo is using it to hide so he can enter the Capulet's grounds. In this way darkness could be seen as something useful. Juliet also says that darkness is useful as it hides the fact that she blushes when Romeo talks to her; 'the mask of night is on my face.'
Light is also used in a different way when Juliet says 'And not impute this yielding to light love' which means she does not want Romeo to think she is taking what is happening lightly.
The coming of the light at the end of the scene marks the end of their exchange and we are left to wonder on what will happen next.
In addition to the examples mentioned in the previous answer, another example of images of light is shown when Romeo compares Juliet's eyes to stars in the night sky. He says: "Two of th fairest stars in all the heaven,/Having some business, do entreat her eyes/To twinkle in their spheres till they return." Romeo is saying that if the stars had to leave their position to attend to something else, they would ask Juliet's eyes to shine in their place because they are as bright and beautiful. Romeo continues this light imagery by comparing Juliet's cheek to the starts saying that the "brightness of her cheek would shame those stars/As daylight doth a lamp;"