There are two clear metaphors in Sylvia Plath's poem "Mirror." Some readers may see an additional metaphor in the use of the word "swallow" in line two, which suggests that the mirror is a creature capable of swallowing things, but this idea is not developed. Other readers may regard the entire poem as a metaphor describing, for instance, a person so passive that she only reflects whatever is before her. However, the poem works just as well, and arguably better, when interpreted as the description of a literal mirror.
The first undoubted metaphor comes when Plath describes the mirror as "The eye of a little god." This image recalls the household gods of the Romans, the Penates, who would be worshipped at small domestic altars. Worshippers look anxiously into the mirror, hoping that it will be kind, but this god tells only the truth.
The second stanza of the poem is an extended metaphor in which the mirror is described as a lake. The strangeness of this metaphor comes from the fact that the lake in question is being used as a mirror. Since the woman is merely looking into the lake to see her own reflection, it is, at first, difficult to see what the metaphor adds to the poem. This question is answered in the dramatic final image, when her reflection rises from the depths of the lake "like a terrible fish." It shows her as an old woman rather than the young girl she imagines herself as.