PLEASE HELPP!!! EASY QUESTION ABOUT POEM THAT I HAVE INSERTED BELOW!!.

Question- What literary device is used in line 12, "But the light did not go out." explain.

Poem-
In her class our speculations
ranged the world;
She aroused us to book
waving discussions.
Every morning we came to her 5
carrying new truths,
new facts,
new ideas - -
Cupped and sheltered in our hands
like captured fireflies. 10
When she went away, a sadness came over us –
but the light did not go out.
She left her signature upon us –
the literature of the teacher
who writes on children’s minds. 15
I’ve had many teachers who taught me
soon forgotten things –
But only a few like her –
Who created in me a new thing,
a new attitude, 20
a new hunger.
I suppose to a large extent,
I am the unsigned manuscript of that teacher.
What deathless power lies in the hands
of such a person. 25