How does author use imagery or figurative language to convey his attitude towards life in "Dover Beach"? Support with evidence from the selection.

The speaker is lamenting the fact that the world is changing. The initial scene is comprised of calm images. The sea is calm, the moon is reflected in the water, and the English cliffs are "glimmering" and powerfully "vast." This visual imagery suggests a world that is marked by peace, beauty, and power. But subsequent lines will describe that world fading into the past. This is/was a world in which faith/religion played a vital role in giving people comfort and a feeling of purpose. 

To signify the initiation of the new, scientific era, the speaker shifts from the calm, visual imagery to some shocking auditory imagery: 

Listen! you hear the grating roar

Of pebbles which the waves draw back, and fling,

At their return, up the high strand,

Begin, and cease, and then again begin,

With tremulous cadence slow, and bring

The eternal note of sadness in. 

This audible symbolism describes the shocking transition from a world comforted by faith to a more scientific world and one which no longer has that comforting feeling. The "grating roar" illustrates an erosion of the sand beneath. The "eternal note of sadness" describes the speaker's feeling with this loss of religious certainty. 

In the third stanza, the speaker notes how the "Sea of Faith" which once fit the world like a "girdle," is now withdrawing. We have the image that the world was once held together (like a girdle) by faith, but now that "sea" is retreating. The world has lost that unifying idea. Without this unity, the world feels fragmented, and this might lead to "ignorant armies" which "clash by night." In the final stanza, the speaker looks to his lover for some kind of meaningful connection to replace the comfort he (and the world) once had.