Science Questions

How does a one-way mirror work?

How does a one-way mirror work? Liana Serbonne

Dave - There isn't actually such a thing as a one-way mirror. It's not possible without using some really exciting electronics. I don't know if you've ever sat inside a room where it's dark outside but light inside the room. If you look at a piece of glass all you can see is the reflection. You can't see outside. That's because a piece of glass will always reflect maybe 10% of the light that hits it. Normally if it's outside and it's brighter the light coming through it completely dominates and so you don't really notice the reflection. You just see stuff going on behind it. If it's really dark outside and there's no light coming from the outside so all you can see is a reflection. Normally, one-way mirrors are semisilvered. There's some silver on the back of them so they're more reflective than a piece of glass. Maybe they'll reflect 80% of the light which hits them so you can see the reflection very strongly. If you're in behind them it's very dark so you can't see that reflection coming through them. You can see people in the light side but the light side can't see you.

Chris - So when you see a detective story drama on telly where you've got these detectives watching someone being interviewed and they're in a brightly-lit room and the person being interviewed is in a brightly lit room, that's a myth.

Dave - That wouldn't work, no.

October 2008