How do 2-way mirrors work?

Maybe you’ve seen in an episode of a cop show of a suspect getting interviewed in a room. This room probably had a two-way mirror, but what are they? These mirrors work with light intensity, basically if 2 rooms are side by side and one of them has a window and the other room doesn’t, its going to be darker on the side without a window. That means that the darker room would see through the glass, and the bright room would be facing a mirror. The people who make the 2-way mirrors have to put a reflective coating on. The coating on regular mirrors is dense and makes the light that hits its surface to reflect. The coating on two way mirrors is more sparse. Meaning that about half of the light reflects or bounces off, while the other half sinks through the mirror like a window.

One popular question that is asked about these mirrors is, “How can I tell if a mirror is two-way?” one test people do is they put their finger right up against the mirror, if their finger has a gap between the fingertips then it is a one way mirror. If there is no gap, that means someone could be watching from the other side.