However, an idea came to Alice when she heard about how Kirito had reflected the light from Fanatio’s divine instrument, the «Heaven Piercing Sword», with a mirror created from just thermal and crystal elements in the fight on the cathedral’s fiftieth floor.

If light did no more than rebound upon touching a mirror—she would only have to produce a fully sealed mirror.

And if she were to generate luminous elements within it.

It could, theoretically, maintain an infinite number of luminous elements until the mirror’s Life expired.

(Alicization Exploding, 115)