Flat mirrors are smooth, highly polished, flat surfaces, for reflecting light. The actual reflecting surface is usually a thin coating of silver or aluminum on glass.
Optical flats or test plates are polished surfaces that are used as references against the flatness of unknown surfaces for comparison. They use the property of interference to measure the flatness of a test surface.
Optical mirrors have a smooth, highly-polished, plane or curved surface for reflecting light. Usually, the reflecting surface is a thin coating of silver, or aluminum on glass.
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