A one way mirror gives the illusion that one side of the glass is a reflective mirror and the other side is normal, see-through glass. On one side, a person looking at the glass will see their image just as in a mirror. On the other side, the onlooker can see whatever is on the other side of the glass.
1. A piece of glass coated with a very thin layer of reflective metal - often aluminium is used. Only half the molecules of the aluminium needed to coat a normal mirror is used. This means that some of the light is reflected and some is allowed to pass through the mirror.
2. Lighting - one side of the mirror must be brightly lit and the other side left in semi-darkness. In the side with the light, the image will be reflected. The darker room will act like a window. If the lights were turned off, the mirror may act more like a window on both sides. If the lights in the darkened room were switched on, they would be seen by the other side.
They are particularly good for observing behaviour without the 'subject' being aware of this. Police interview rooms and behavioural experiments would use them.
The windows of buildings and cars are sometimes made from one way mirrors. The bright sunlight from outside is the reflective side. Turning the lights on at night could make the occupants visible.
Other uses include theatrical illusions. Teleprompter also use this technology. This is where a presenter's script appears on the glass on one side so that the presenter can see and read it. The camera does not see the script. The camera will be in relative darkness whilst the presenter is lit.