How can the good guys see through the mirror while the bad guy sees nothing but a shiny surface?
The mirror used in your favorite detective show has a thin layer of partially reflective coating. Only about 80 percent of the light is reflected, while 20 percent passes through. Importantly, the suspect’s room is about 10 times brighter than the spying detectives’ room, so the light that passes through the mirror is enough to give the detectives a good look at the suspect.
Because the detectives’ room is not as bright, the scant amount of light that passes through into the very bright room is not enough to create an image.