In this brilliant poem by Sylvia Plath, the theme of old age is explored. One of the key devices of the poem is the speaker or persona of the poem that Plath has chosen to explore this theme. The mirror sees the woman examining herself in her day by day. We know that the mirror thinks that it is precise in its judgement - the very language that the mirror uses shows how matter-of-fact the mirror is in its judgement: "I am silver and exact." Yet interestingly, although it says it is "unmisted by love or dislike" we come to doubt this later on when the mirror refers to the moon and the candles as "liars" - the mirror appears to be jealous when her owner prefers to see her reflection cast in their softer (and less cruel) light.
The mirror now uses a metaphor to describe itself as a lake - this allows us to imagine the desperation of the woman who is faced with the inevitable process of ageing. The woman searches the reaches of the lake "for who she really is." It is the last two lines that contain the crux of the poem:
In me she has drowned a young girl, and in me an old woman
Rises towards her day after day, like a terrible fish.
A frightening image, but also one that perhaps reflects the dangers of vanity - the woman has "drowned" herself in the lake of the mirror, and now faces the inexorable fact of old age - imagined as a "terrible fish" - we are left to reflect that if the woman had not spent so much time agonising over wrinkles, she might have had a happier life and wouldn't have wasted her youth.