Quintus is standing in a hallway facing a mirror hanging on the wall, with his back to the opposing wall. A few steps behind him, Octavia is approaching from around a corner in the hallway, which Quintus cannot see directly. Without turning around, can Quintus tell when Octavia has entered the hallway, and how does he know?

Provide a very brief response.
Quintus can tell when Octavia has entered the hallway by seeing her reflection in the mirror as she approaches from behind him. The mirror provides a reversed view of the scene behind Quintus, allowing him to see objects or people that are otherwise out of his direct line of sight. When Octavia comes around the corner and into the reflective range of the mirror, her image will be visible to Quintus without him needing to turn around. The reflection functions similarly to a periscope, bending the line of sight around the corner to reveal what's not directly in view.