One-way mirror

A partially reflective pane that appears mirrorlike from its brightly lit side while allowing a view through from its darker side. Its effect depends on a strong difference in illumination between the two sides.

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Partial reflector: An optical surface that reflects part of incident light and transmits the rest. A one-way mirror is a partial reflector, not a surface that blocks light in one direction.
Police lineup: A procedure in which a witness identifies a suspected offender among several people. One-way mirrors have been used to let witnesses observe lineups without being seen.
Frosted glass: Glass with a textured or treated surface that scatters light and blurs images passing through it. It diffuses transmitted images instead of masking them with a reflected scene.
Visual privacy: The ability to limit who can see a person, activity, or space. The pane’s apparent privacy can fail when the two sides are similarly illuminated.
Specular reflection: Reflection from a smooth surface that preserves the directional organization of light. Its reflected image comes from orderly surface reflection.
Observation room: A room designed for monitoring people or activities from a separate viewing area. The pane can separate observers from the people being monitored.
Smart glass: Glazing whose light transmission or opacity can be changed electronically or by an applied stimulus. Unlike a one-way mirror, its privacy effect can be switched without changing room lighting.
Surveillance: The systematic observation of people, places, or activities for information or control. One-way mirrors can conceal observers while enabling direct visual monitoring.
Transmittance: The fraction of incident light that passes through a material or optical surface. The view through the pane depends on its transmitted light.
Behavioral observation: Systematic recording of behavior in natural or structured settings. Researchers have used screened observation rooms to watch behavior without direct interaction.