History of Two-Sided Mirrors

• Many people know about two-sided mirrors from crime shows. The police sit in one room, while the interrogator and criminal sit in another. To the criminal, there appears to be a mirror on the wall. The police, however, can see through the mirror as if it were a window. At first glance, it would seem that the two surfaces of the mirror are coated with different substances. In truth, the mirror is exactly the same on both sides. Here's how it works.
  
  

Composition of the Mirror

• A two-sided mirror is composed of a piece of glass covered by a very thin reflective coating. In fact, it is so thin that it is called a half-silvered surface. The coating reflects back about half of the light that hits the mirror's surface, while letting the remaining half of the light pass through.
  
  

Lighting

• A two-sided mirror only works if the lighting in the two adjacent rooms is set appropriately. One room must be very well-lit, and the other one must be very dark. This creates an imbalance of light traveling between the two rooms.
  
  

How It Works

• Half of the light from the bright room is reflected back at observers in the bright room. Therefore, the observers see their own reflections. While half of the light in the dark room is reflected back as well, that still adds up to very little light. In addition, half of the bright room's light has passed through the mirror. Therefore, observers in the dark room can still see through the mirror.
  
  

Conclusion

• Based on this information, if the light in the bright room is dimmed, or if an additional light is turned on in the dark room, the mirror would not appear as a mirror at all. The same amount of light would pass through the mirror as would be reflected. Therefore, observers in both rooms would view the mirror as a window. They would be able to see straight through it.