Friday August 31, 2018 Q&A

One-way mirrors let you see people without them seeing you because:


1 they are porous

2 they are ultra-thin

3 they reflect only part of the light that hits

Correct answer: 3. The light that hits them is not fully reflected – part of it passes through the mirror.

Conventional mirrors are made by depositing a thin layer of a reflecting metal like aluminum or silver on one side of a piece of glass. A layer of copper or lead is also typically deposited on top of the reflecting metal to prevent it from oxidizing and to make the mirror completely opaque.

When you look in a conventional mirror, you see your reflection because light passes through the glass and is reflected by the metal. But in a one-way mirror, only part of the light is reflected–the rest passes through the aluminum or silver. That means you can see through the mirror.

Teleprompters, rear-view mirrors and…hidden cameras?

One-way mirrors are most commonly used in interrogation rooms at police stations. Police officers standing behind the mirror can see what goes on in the room, but suspects inside the room can see only their own reflection. However, that only works if the interrogation room is more brightly lit than the area behind the mirror – otherwise, the mirror effect will be lost.

One-way mirrors are also used in teleprompters. The mirror is placed at a 45° angle in front of the camera lens with the reflecting side down, and the text is placed horizontally below the mirror. That enables the speaker in front of the camera to see the reflected text without disturbing the camera shot. Rear-view mirrors in cars are another application: one-way mirrors are what let you reduce the glare from the headlights of cars behind you at night. And it’s worth bearing in mind that someone could hide a camera behind a one-way mirror and film you without you knowing it.