Most of the time this is simply a polished surface: a hard acrylic face over flat colour, carrying one broad highlight that travels as the plate turns. The structure is underneath, and it stays hidden. A bed of glass microspheres returns light along the ray that arrived instead of mirroring it away, so it answers only a light standing where you are — which is why a plate is unremarkable by day and lights up in headlights at night. Bring the light back to your eye and it wakes.