In the poem Dover Beach, the poet uses conflicting imagery to give meaning to the poem. The differences in the way that the poet sees the relationship between the beach and the sea and the way that most people would see it become more pronounced as the poem develops. He also uses the change in attitude from the first stanza to the last to emphasize his message.
The poem starts with the normal image one would expect of a beach and a peaceful moonlit night, but quickly moves to an entirely different point of view. By the end of the first stanza the sea is no longer peaceful and calm, but crashing with a "grating roar". The poet has taken an image that most people associate with tranquility and turned it into a depressing scene. The stanza ends with "The eternal note of sadness" being brought in by the sea. The poet is comparing the sea to the sum of all human troubles. The sea is eternal just as human suffering is eternal. The sea has also seen all of the human suffering and in it's roar the poet can hear that suffering.
When the poet talks about Sophocles and the Aegean he is clearly reinforcing the idea of the sea being the bearer of misery. The reference is to Sophocles tragic plays and the suffering that necessarily accompanied them. This image becomes powerful as the reader realizes that the poet is saying that he can hear the same message on Dover Beach that Sophocles heard so many years ago by the Aegean. He is basically saying that the nature of life doesn't change. There was suffering in the times of the Greeks, suffering in his time, and there will be suffering after he is gone.
The poet finishes the poem of with several images that lend even more power to the poem. At the end of the poem the sea has become the exact opposite of what it was at the beginning. No longer calm, the image the poet uses to describe it is that of two armies senselessly fighting. There is no point to their struggle, just as there is no point to the crashing of the sea. It just is. And that is the point that the poet really seems to be trying to make; that no matter what happens misery and suffering will always be present in the world and there is no explanation for them. Just as the sea will continue to crash on the beaches of the world for eternity.
In Dover Beach, the poet successfully uses contrasting imagery to bring out the meaning of the poem. The change in attitude toward the sea and beach that takes place between the beginning of the poem and the end of the poem is dramatic. It is this change in attitude that allows the reader to decipher the meaning of the poem. All in all a little bit of a depressing poem; the idea that suffering is eternal just as the sea is eternal is a powerful but depressing image.
Source: ChinaStones - http://china-stones.info/coursework/hope-is-pathetic.php