Coated with a thin layer of aluminum, this special glass works like both a mirror and a window. It reflects some light, and lets some light pass through. When your side is brighter, it’s easier to see your own reflection. When your friend’s side is brighter, you see more of their face.
Sometimes called two-way mirrors, glass windows of this type allow viewers on the more dimly lit side to observe people on the other side without detection. They’re used in market research and psychology studies, interrogation rooms, for security observation, and also in the filming of reality TV shows.