Certainly a rather melancholy poem, critic Jo Gill notes that the poem "Mirror" is "catoptric"; that is, it describes while it represents its own structure with two nine-line stanzas that are balanced and dual in nature. While the mirror reflects objectivity--the opposite wall that is "pink with speckles"--at the same time, it is false in its superficial reflection of mere physicality when the woman peers into it as a measure of herself. And, it is this falseness that is represented by the terrible fish that arises from the mirror, that unadultered self that does not conform to mere reflection.
Thus, the theme of Plath's poem arises from the duality of the mirror. While it reflects a true superficial image, it cannot define a person. It is the inner person that one must cultivate on her own, defining oneself by one's own values, not as a reflection of the values of others. Herein lies the theme: No mirror can report the soul, the essence of a person; it can only reflect the visage of a person, a visage which at times may be false, "a terrible fish."
Now I am a lake. A woman bends over me,
Searching my reaches for what she really is....
I am important to her.
The person in the mirror searches where she can find no answers, for the soul does not emerge from the search for self. It is, however, reflected in the heart. This is surely the theme.
Sylvia Plath's Mirror explores some themes. Most importantly, the theme appearence is brought up. The title itself is combined with this theme since mirror is an object desighed to see the apperance. The mirror, which is personified to a degree of making it the narrator, proclaims about a woman's action of checking out her appearence regularly and his unbiased expression of her apperence. The trancience in appearence is another issue brought up.
Searching my reaches for what she really is.
Woman and feminity is another theme ellaborrated. The only character apart from the mirror is this woman. the poem revolves around her feminine action.
Tlme too can be taken as a key theme. Sylvia plath explains about aging, and implies about the march of time which brings the inevitable change ti this woman. The poem talks about this woman's resemble action of looking through the mirror in her youth and ultimately, in her old age.
In me she has drowned a young girl,
and in me, an old woman.
The reader is given clues about two types of transformations in the poem. One is, the transformation of mirror into a narrator and then a lake. The other is the transformation of the woman's age, from yuth to old age. It is one of the themes in relation to this poem.