Plath's "Mirror" has been widely studied for several literary devices including metaphor, personification, allusion and imagery. Each give suggestion at meaning.
Personification takes it's form in the direct metaphors of "I am silver and exact(alluding to a mirror)" and "Now I am a lake". In both objects, she choose to be reflection. As each metaphor develops, the mirror tends to reflect judgmental truth ("The eye of a little god"), while the lake seems to have the ability to change her:
In me she has drowned a young girl, and in me an old woman/Rises toward her day after day, like a terrible fish.
The malability of the lake shows the affect of time on the aging woman.
Imagery occurs in these lines:
Most of the time I meditate on the opposite wall.
It is pink, with speckles. I have looked at it so long
I think it is a part of my heart. But it flickers.
Faces and darkness separate us over and over.
It is important to remember that the speaker here is a mirror. Readers get the visual image of the non-stop image that the mirror sees because of its position in the room, yet faces and the darkness of night appear highlighting the course of time which may feel monotonous.
Allusion to Plato's The Republic occurs in the line "on the opposite wall". This references an image of cave-dwellers who could see shadows on an opposite wall because of the light of fire.
The content of the poem is another aspect all together. When taken as a whole, these descriptions of mere objects are so intense and so specifically chosen that readers cannot help but notice the Narcissistic concept at work. Plath relished in the beauty of her poetry, but the intensity of a mirror's power to the human mind is something that must have struck her personally. When people look in mirrors they either grow conceit for the beauty they see in themselves, or the disappointment of imperfection. The latter is more likely Plath's perception with the convictions of truth she alludes to in the phrases "just as it is", and "only truthful" as she characterizes the mirror.
Furthermore, we see time at work in the end of each stanza which might mean she was anticipating the end of life. All of Plath's poetry is considered highly autobiographical and this poem is consistent with her life.