A security technician is setting up a laser-based alarm system. The system relies on a narrow, intense laser beam reflecting off a highly polished silver mirror to reach a detector on the opposite side of the room.

However, after several weeks of operation, a layer of fine dust accumulates on the mirror. The detector stops receiving a strong signal, and the technician notices that instead of a sharp, concentrated spot, a faint, blurry glow is visible on the wall around the detector.