Petrarch's first idea -- that Laura remains beautiful and...
In Sonnet 90, Petrarch expresses two main ideas about love: one is that his love for Laura does not fade over time and as she ages; the second is that his love for her is painful because it is not returned.
Petrarch's first idea -- that Laura remains beautiful and desirable to him over the years -- is conveyed mostly through physical descriptions of Laura, using imagery and figurative language. Stanza one (lines 1-4) provide a good example of imagery:
Upon the breeze she spread her golden hair
that in a thousand gentle knots was turned
and the sweet light beyond all radiance burned
in eyes where now that radiance is rare
Here, Petrarch describes Laura's hair blowing in the wind in a way that allows readers to picture the image. This excerpt also ends with a contrast -- Petrarch notes that Laura's eyes no longer show the same "radiance" they did when she was younger. Petrarch continues to discuss his physical attraction to Laura in lines 9-13:
She moved not like a mortal, but as though
she bore an angel's form, her words had then
a sound that simple human voices lack;
a heavenly spirit, a living sun
was what I saw
Here, Petrarch uses figurative language to compare Laura to an angel and place her on a pedestal above other women. He equates Laura to "a heavenly spirit, a living sun," and these metaphors indicate how highly Petrarch thinks of her.
However, Laura never returned Petrarch's love, and the pain he feels as a result can be seen in the middle and at the end of the sonnet. In lines 5-8, Petrarch writes:
and in her face there seemed to come an air
of pity, true or false, that I discerned:
I had love's tinder in my breast unburned,
was it a wonder if it kindled there?
In this stanza, Petrarch perceives in Laura's face "an air/ of pity," so even though he is not sure what she feels, he interprets Laura's look to mean that she feels sorry for him (likely because she will never love him and he so obviously admires her). Petrarch uses a metaphor in lines 7-8 to describe the passion he feels for Laura: he has the "tinder" to begin the fire of love, and it is "kindled" because Petrarch feels attracted to Laura. However, the tinder is "unburned" because it is not shared by Laura. Finally, at the end of the sonnet, Petrarch writes,
now, if it is not so,
the wound's not healed because the bow goes.
He feels injured or damaged because Laura does not return his love. The "wound" can never be healed, unless she were to love him. In Sonnet 90, Petrarch uses imagery and figurative language to both express his continuing love for Laura and the pain he feels because that love is unrequited.