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A modern (Korean War era and later, at least) signal mirror has a hole in the center, and the hole is surrounded by a grid of retroreflectors. You aim the mirror spot at something close by, then lo...
Answer
A modern (Korean War era and later, at least) signal mirror has a hole in the center, and the hole is surrounded by a grid of retroreflectors. You aim the mirror spot at something close by, then lo...