One-way mirror

Reflect me if you can!

20 minutes


  • Carefully review the general safety advice in the instruction book before starting the experiment.
  • Read the "Working with batteries" section of the safety guidelines carefully before proceeding. Always disconnect the setup after finishing the experiment.
  • Keep new and used coin cell batteries away from children.
  • Disassemble the setup after the experiment.

Step-by-step instructions

Build a model of an observation room.


Put the characters in their places and designate which room is which.


Now close the “doors.”


Next, separate the good guys from the bad guys with a mirror just like in the movies!


Connect the LED to the battery and use it as a light source. Turn the battery over if the LED does not light up.


Finally, illuminate the suspect's room. Observe how differences in room lighting can serve to conceal and reveal the truth!



  • Dispose of used batteries in accordance with local regulations.
  • Dispose of solid waste together with household garbage.

Scientific description

A typical observation room in a detective movie often features a large wall mirror. This is no ordinary looking-glass—it’s a one-way mirror! What’s the difference?

In general, a mirror is a piece of glass with an even layer of a reflective coating. A one-way mirror’s reflective layer is intentionally designed to transmit 10–20% of light. How does it work one way and not the other? The secret is in how the two rooms are lit relative to each other! The officer’s  room is almost pitch black, while the suspect’s  room is brightly lit.

From the suspect’s  position, 80–90% of the bright light in his room is reflected back at him by the one-way mirror, which obviously prevails over the 10–20% of transmitted dim light  from the observation room. That’s why the suspect can only distinguish his own reflection.

The officer’s  room, however, is kept so dark that even 90% of its reflected light is less than 10% of the bright light transmitted from the suspect’s room through the one-way mirror! Thus, the officer’s eyes can easily distinguish the light transmitted from the suspect’s room, and everything going on inside.