The secret of one-way glass is that it only works under certain conditions. One side of the glass is covered with a super-thin reflective layer, called a half-silvered coating. It’s so thin that about half of all light hitting its surface is reflected while the rest is let through.
When placed between two rooms – one brightly lit and one dark – the mirrored side has plenty of light to reflect, giving it the appearance of a normal mirror.
On the dark side, however, there’s enough light allowed through by the half-silvered surface to see the other room perfectly. The effect is ruined if the lights are switched on in the dark room, because the surface isn’t opaque so the light, and occupants, can be seen from the other side.