Arnold's poem, Dover Beach begins with a naturalistic description of the nightly Dover sea-scape, with a series of images of sight and sound, of light and darkness: the calm sea full to the brim; the moon on the straits as personified in 'lies fair'; the light on the French coast, gleaming, and then gone; the Dover cliffs standing 'glimmering and vast'. The fullness of the tide & the calmness of the sea suggest a contrast to the moral-spiritual vacuum as much as to the state of unrest in Arnold's contemporary Victorian England that the elegy aims to lament. If the gleaming light on the French coast is a sign of hope, its fast disappearance suggests the gloom of hopelessness. The vast & glimmering cliffs of Dover furthers the suggestion of dimness.
The visual image of 'the long line of spray', of the sea-water lapping on to the shore--'the moon-blanched land' again highlights the mingling of light & darkness. This is followed by the aural image of 'the grating roar of pebbles'--a harsh noise born of the friction of pebbles, developing with its 'tremulous cadence slow' into 'an eternal note of sadness'. The audio-visuality of the image leads to the theme of universal sadness. The lyrical self projects his own feelings of melancholy on to the sights and sounds of the sea-scape.
In stanza 3, Arnold offers the image of the symbolic-allegorical sea--the 'Sea of Faith' which, unlike the Dover sea, retreats from the shores of human life. There was a time--presumably the Middle Ages--when religion was not overshadowed with doubt, and the the 'Sea of Faith' was a protective girdle--'Lay like the folds of a bright girdle furled'. But now the poet hears the long melancholy noise of the withdrawing 'Sea of Faith', the sound of the retreat of faith colaterally producing a vision of the dark, dreary, naked reality of the human world.
The final stanza presents the conflicting images of sights & sounds, of light & darkness: the illusory look of a beautiful world lying before us as 'a land of dreams' when in reality the world has turned into 'a darkling plain' where only 'ignorant armies clash by night'. The bright & beautiful world is but an appearance. It's a veritable battle-ground swept with sounds of hostility and blind aggression.