Light is often used as a metaphor for knowledge. In this play, it is also often a metaphor for love.
The sun is a constant presence in the play. Benvolio refers to “the worshipp'd sun” and Mercutio to the “all-cheering sun” (1:1).
Romeo’s reference comparing Juliet to the sun is one of the most famous in all of Shakespeare.
But soft! What light through yonder window breaks?
It is the East, and Juliet is the sun!
Arise, fair sun, and kill the envious moon… (Act 2, Scene 2, p. 38)
It should be noted that Romeo also compares Rosaline to the sun.
One fairer than my love? The all-seeing sun
Ne'er saw her match since first the world begun. (Act 1, Scene 2, p. 21)
Romeo clearly cannot think of anything more beautiful than the sun, except the girl he is currently in love with. Yet in this case, comparing Juliet to the sun is a metaphor for Romeo’s return to happiness. He has been in a deep state of depression since Rosaline left him. Now he sees the light.
Juliet’s reference to the sun is different. She refers to the sun as driving out the darkness.
Love's heralds should be thoughts,
Which ten times faster glide than the sun's beams(5)
Driving back shadows over low'ring hills. (Act 2, Scene 5, p. 55)
Again, love is conquering misery, as light conquers darkness.
While the sun gets a lot of appreciation in the play, the poor moon really gets insulted. When Romeo swears on the moon, Juliet objects.
O, swear not by the moon, the inconstant moon,
That monthly changes in her circled orb,
Lest that thy love prove likewise variable. (Act 2, Scene 2, p. 41)
Of course Romeo already talked about the sun killing the “envious moon” as well. The moon has served as an alternative to the sun—to love—so Juliet does not want Romeo to swear by it.
Romeo makes an interesting joke about a torch on the way to the party.
Give me a torch. I am not for this ambling.
Being but heavy, I will bear the light. (Act 1, Scene 4, p. 26)
Although the pun is funny, it is also a metaphor. Romeo has been depressed, having no love, and seeks the light of the torch, love. He also mentions that a torch has nothing on Juliet, when he says she “doth teach the torches to burn bright” (1:5).
Lightening is also light, but it is quick light. It refers to poor choices about love, or quick love.
Although I joy in thee,
I have no joy of this contract to-night.
It is too rash, too unadvis'd, too sudden;
Too like the lightning, which doth cease to be(125)
Ere one can say ‘It lightens.’ (Act 2, Scene 2, p. 41)
Juliet worries that her love for Romeo might be too fast, like lightening rather than a lasting love.
Shakespeare was clearly fond of light and dark metaphors, so light is used in many ways to refer to the different kinds of love in Romeo and Juliet.