How does the phrase “a wave of enthusiasm” apply a natural image to an emotion?

The phrase "a wave of enthusiasm" functions as a metaphor, a trope that compares two ostensibly dissimilar phenomena to create a vivid rhetorical effect.1

By applying the natural image of a "wave" to the abstract concept of "enthusiasm," the speaker utilizes the physical properties of water—such as its movement, force, and ability to spread—to describe an emotion. This comparison suggests that enthusiasm is a powerful, surging, and collective force that can sweep through a group of people, much like a physical wave moves across the surface of the ocean.

In the context of rhetoric, this figure of speech serves as a tool to shape the audience's perception, transforming an internal feeling into a dynamic, observable event. By grounding the emotion in a familiar natural phenomenon, the speaker makes the intensity and contagious nature of the enthusiasm more tangible and impactful for the listener.1