The phrase "my heart leaps up" in the first line of the poem is an example of metaphor. The narrator's heart does not literally leap up, as it is fixed in place inside his body. Instead, it perhaps beats harder. In saying, "my heart leaps up," the narrator is comparing the joy he feels at seeing a rainbow to the sensation of a leaping heart.
Wordsworth uses imagery in the second line of the poem when he writes of a "rainbow in the sky." We as readers can visualize the beauty of a colorful rainbow cutting across the sky.
The poem also uses rhyme to create a sense of rhythm, such as in "behold/old" and "began/man."
Parallelism appears in the repetitive structure of the three lines that deal with past, present, and future: "So it was . . . So it is . . . So be it." This parallel structure reflects and reinforces the idea that the poet's attitude toward the rainbow will never change.
The poet also uses exclamation when he writes "Or let me die!" The exclamation point emphasizes how strongly the poet feels about finding joy in a rainbow. He would rather be dead than oblivious to the sight.