What is the theme of Wordsworth's "The Rainbow?"
In this poem, the speaker says that his "heart leaps up" whenever he sees a rainbow overhead—in other words, he feels tremendous joy. This is how he responded to the sight when he was a child, "when [his] life began." It is the same now that he has grown, and his heart will continue to feel this way even when he is an old man. He would rather die than no longer respond to the rainbow in this way.
The last few lines are a bit more difficult to understand: Wordsworth employs a paradox when he says that the child is the father of man, and he means, perhaps, that the child actually knows more than the grown-up. There is no question that he responded to the rainbow with great joy as a child, but there exists the possibility that he will not continue to respond this way as he ages: we might understand this to mean, then, that the child is more perceptive and more knowing than the aged person. It seems that the piety he speaks of is more like the idea of a reverence for Nature as something divine, rather than traditional notions of religious piety. He hopes, in the end, that his days will be filled with this sort of reverence for the natural.
As far as theme, the poem conveys the idea that Nature can inspire all of us, no matter the age, to feel great joy. Further, the joy inspired by Nature is something that many of us lose as we age. Finally, the poem illuminates the idea that children understand Nature as a source of joy better than adults do.
William Wordsworth was a Nature poet who worshipped Nature as his God and it was his main source of spirtual comfort and escape from all the cares of this world. His association with life giving and life sustaining Nature began even when he was only a child and remained with him till his death.
In this short lyric, the 'rainbow' symbolizes the life sustaining and life nourishing goodness of Nature. The sight of the beautiful rainbow which he saw when he was only a child is deeply etched in his memory and the same joy that he experienced when he saw it as a child contiunes to remain with him through his adulthood. He desires that this same childhoood joy should continue to sustain him even in his old age. Wordsworth says that he would rather die than not being able to experience the same joy that he experienced when he saw the rainbow when he was a small boy after he becomes an old man.
The memory of the beautiful rainbow and its pleasant associations form the link between his childhood, adulthood and his old age:past, present and future. Wordsworth concludes the poem by expressing the desire that each day of his existence be linked with the next by beautiful and simple natural sights like the rainbow.
For Wordsworth the life nourishing and life sustaining memories of beautiful natural sights like the rainbow are very precious and he deeply desires that they link each day of his life on this earth and remain with him till his death.