There is actually no such thing as a true one-way mirror; if there was it would violate the laws of physics. The difference between a one-way mirror and a regular mirror is the reflective coating. The one-way mirror has a very, very thin layer compared to that of a regular mirror. This causes only about half of the light to be reflected back from the mirror while the other half passes through the glass. Since half of the light doesn’t get reflected and passes through the mirror, there has to be special light conditions to get the mirror to act as a one-way mirror. If you have the mirror separating two rooms, one room has to be brightly lit while the other needs to have very little light. The room with the bright light will see their reflection when they look into the mirror because there is a sufficient amount of light that can be reflected. The room with the dim lights will be able to see straight through the mirror like a window. There isn’t a sufficient amount of light in the dim room to create a reflection.