The physics of seeing someone who can't see you

We may earn a commission from links on this page.

Everyone's seen the police shows in which they shuffle suspects into a room with a mirror on the side. How do the police see in while the suspect doesn't see out?

The mirror is covered with a light silver film. This film acts as a kind of grating. It lets about half of the light that hits it go through, while sending half back. From then on, it's just the lighting. Inside the interrogation room, the lights are bright. That sends a flood of light to the mirror, half of which gets turned back on the people in the room. Outside the interrogation room, the lights are dimmed. Only a trickle of light makes it to the mirror, and half of that comes back.

Light still makes it through to either side, but seeing the dimmer light is like trying to see the stars during the day. The light from the sun is so bright that their minor lights are lost amid the glow. If someone were to cut the lights in the interrogation room, the subjects inside would see the dim light, and the people standing there, clearly.


The two-way, or one-way, mirror is also used in haunted houses. By subtly adjusting the lighting and keeping the space behind it blank, people can make it look like a regular mirror. Then, after you think of it as one, another person wanders through the other side, you can dimly see their image, and think of them as a ghost.

Still no word on the two different names, though.

Via eHow and How Stuff Works.