How do one-way mirrors work?

One-way mirrors are most commonly used in interrogation rooms, teleprompters, and security monitoring (usually to hide a camera or make us perceive a camera is present). The premise of one-way mirrors is to allow a person on one side to see through it while a person on the other side sees only a reflection. Unfortunately, there is no such thing as a one-way mirror per se, but the illusion works with the help of lighting. To create this illusion, a piece of glass is coated with a very thin layer of reflective metal (e.g. aluminum); this surface is often called half-silvered. A half-silvered surface will not reflect nearly as much light as a thickly coated surface, but it still reflects around half of the light that hits it. This way, if one side of the half-silvered surface is brightly lit, then enough light will reflect off of it to make one's reflection appear on that side. The other side of the glass is often kept very dark, allowing a viewer to see through to the lit side.

To better understand how light creates a one-way illusion, try this test. Look out a window when it is sunny outside and dark inside. Notice how easily you can see through the glass and observe what is happening outside. Now, look out the same window at night when it is dark outside and your house/office is brightly lit. Do you see your reflection? Can you still see what is going on outside? If you can grasp the importance of lighting using just plain glass, then you can easily understand the difference that a thin reflective layer (i.e. half-silvered) makes in creating the one-way mirror illusion.