The eyes in Night are a complex symbol that carries through from the beginning of the narrative when Moishe the Beadle talks about what he witnessed through to Elie's ending comment about the eyes that stare back at him from the mirror. What might be called the primary underlying element of the symbol is that the eyes are a witness.
Francois Mauriac, in his Forward, describes Beadle's efforts to tell what his eyes witnessed so as to warn the other inhabitants of Sighet. This symbolically equates eyes with a witness. While we all know an eye-witness must see a thing, the symbol isn't usually carried so far as to equate eyes with witness. Generally, the whole individual is the witness, whereas the eyes of those seeing the Holocaust are the witness, even if the individual cannot speak of it, cannot understand it, cannot live with the memory of it.
[the] pleas of a witness who, ... related to them what he has seen with his own eyes, but they refuse to believe him ....
The second part of the symbol is that the eyes reveal what the human is. Wiesel first wrote Night in Yiddish. In the Preface to the new translation of the English version of Night, Wiesel quotes a portion of the Yiddish original. The quote explains that in the eyes of each Jewish person resides the image of God. It is, therefore, the image of God the eyes reveal:
We believed in God, trusted in man, and lived with the illusion that everyone of us has been entrusted with a sacred spark from the Shekhinah's flame; that every one of us carries in his eyes and in his soul a reflection of God's image.
The symbol carries another level: the eyes see God reflected (they both show forth God's image and see God reflected from other humans). Elie relates that when the camp witnessed the hanging of a child, a man in the crowd moaned and asked, "Where is God?" while a silent voice inside Elie formed an answer, saying that God was hanging with the child:
"For God's sake, where is God?"
And from within me, I heard a voice answer:
"Where He is? This is where--hanging here from this gallows."
The child--with the image of God in his eyes and God reflected from those eyes to others--hung, with God--both on the gallows--while "tears, like drops of wax, flowed from [the child's] eyes."
This complex symbol establishes the eyes as a witness, as the image of God, as the reflection of God, as the tears and lost joy of God, while also being the tears and lost joy of the Jewish people: "the joy in his eyes was gone." Remembering that, for Wiesel, God hung with the child, the complexity of this symbol is confirmed with Wiesel's closing comment about his own eyes:
From the depths of the mirror, a corpse gazed back at me. The look in his eyes, as they stared into mine, has never left me.
At the end of the memoir there is a quote that might help you answer this question. Elie states, "From the depths of the mirror, a corpse gazed back at me. The look in his eyes, as they stared into mine, has never left me." Elie is describing the way he feels when he looks at his transformed face for the first time since he entered the camps. Elie goes through a physical, mental and religious transformation throughout his time in the camps.
Physically, he has been beaten, starved, dehumanized, and worked to the bone. Mentally, he has been separated from the female members of his family, and he has watched his father die. Religiously, he has question how God could let this horrible event occur.
His eyes reveal a transformed, brokendown person. When he enters the camps he is young, healthy, loving and religious. When he leaves the camps he is broken.