The central theme in the poem "Mirror," by Sylvia Plath, is self-reflection and appearance. The author uses imagery, diction, and poetic devices to convey this theme. Imagery of water, personification, point of view and shift are applied to emphasize Plath's subject. Imagery and diction are very important tools employed throughout "Mirror". Water and water related diction are repeatedly mentioned in the poem to represent clarity and purity. This reinforces the theme of appearance because with the clarity of a mirror, one can view oneself with ease. The mirror is described as "unmisted by love or dislike," meaning it is totally unbiased. The mirror "swallow[s] immediately" anything it sees, much like how a body of water can swallow an object up. The water imagery continues in the second stanza when the mirror becomes a reflective lake. The woman "drowned a young girl, and in me an old woman Rises toward her." This is, of course, a metaphor for the aging of the woman. In the last line of the poem, Plath writes that the old woman that reflects back is "like a terrible fish." She uses this simile to illustrate the negative side of the total honesty of the reflection. .
The mirror is often personified in this poem. The poet writes that the mirror looks and sees things, when in fact it cannot. In the first stanza, the mirror "meditate[s] on the opposite wall," and it has "looked at it so long  it is part of [its] heart." The reader discovers that the mirror feels some sort of emotion, additionally personifying it. The mirror "faithfully" reflects back the image of the person looking at it. As an inanimate object cannot be faithful, this personifies the mirror as almost feeling pride in what it does. Personifying the mirror supports the theme of appearance because it transforms the physical mechanism of light bouncing off a mirror into our own eyes into a being that tells us the truth about how we look.