A one-way mirror seems to be a mirror when seen from one side, but as a window when seen from the opposite side. Thus the window is disguised as a mirror to allow secret surveillance. Physically, there is no such thing as a one-way mirror. That is, the amount of light reflected from one side is the same as that reflected from the other. The light transmitted in one direction is equal that transmitted in the opposite direction. How then does a one-way mirror work? First, the mirror isn’t totally reflecting. It transmits half the light and reflects the other half. The second requirement has to do with lighting. It is imperative that the observation room remain dark, because if a lamp were turned on, some of that light would pass through into the interrogation room as well.