Can You Spot a Secret Mirror with This Easy Test? A one-way mirror, also known as a two-way mirror, works based on the principles of light reflection and transmission, specifically the way light interacts with a partially reflective surface. It is a sheet of glass coated with a thin, semi-reflective layer of metal. This coating allows some light to pass through while reflecting the rest. The key to making the mirror appear one-way lies in the lighting conditions: the room on the reflective side

How Do One-Way Mirrors Work? (aka Two-Way Mirrors) | Howstuffworks