How two way mirrors work
In a regular mirror, you have a sheet of glass and a thin layer of reflective metal (usually aluminium) that is sprayed onto the glass. There is also a dark, light absorbing layer on the back of the mirror so that no extra light beams escape. With two way mirrors, this thin layer of reflective metal is made even thinner and the light absorbing layer is taken away. This causes some of the light to reflect, and some of the light to travel right through the mirror. Although the reflective layer is very thin, the two way mirror is still able to act as a regular mirror in certain conditions.
For a two way mirror to work, one side must be darker (observing side) and one side must be brighter (interrogating side). Without these lighting conditions, neither side would be able to see through the glass very clearly. Since the cop's side is darker, more light comes through the mirror than is reflected. Since the interrogation side is brighter, more light reflects than travels through the glass. This is why the cops can observe while the suspect only sees his/her reflection.
For a two way mirror to work, one side must be darker (observing side) and one side must be brighter (interrogating side). Without these lighting conditions, neither side would be able to see through the glass very clearly. Since the cop's side is darker, more light comes through the mirror than is reflected. Since the interrogation side is brighter, more light reflects than travels through the glass. This is why the cops can observe while the suspect only sees his/her reflection.