In Greek mythology, Athena gave Perseus a polished shield for facing Medusa safely. Its reflective surface let him observe Medusa indirectly rather than looking directly at her face.
Direct sight of Medusa could turn a living person to stone. By watching the reflection in the shield, Perseus approached while avoiding her petrifying gaze and then used his harpe to sever her head.
The shield is often described as bright, polished, or reflective rather than as an ordinary battlefield shield. Some later artworks show Perseus using a mirror-like surface, but the essential mythic idea is indirect vision. Athena’s assistance also connects Medusa’s head with the goddess’s later aegis imagery.