Further informationson the exhibit "Spy in the mirror"
This is a so-called one-way mirror or spy mirror. This means that incoming light is largely reflected on one side and allows only a small portion to pass through, while the incoming light is completely absorbed on the other side. This allows you to observe the people on the opposite side, while your counterpart can only see his own reflection.
A medium that has such characteristics is not technically possible. In order to produce glass panes with (largely) these qualities, a weakness of the human sense of sight (or of all optical recording devices) is used: Only a limited range of the existing brightness spectrum can be captured, while other ranges appear overexposed or underexposed.
In this case, the mirror is coated with a special thin metal oxide layer so that only a certain part of the light is reflected and the rest penetrates to the other side. To make this effect possible, the room to be observed must be very bright, but the room from which it is observed, here the cabin, must be very dark.