How do one-way mirrors work in stores?

[Silverbolt]

Question

In many department stores, large plane mirrors have been placed high on walls or on projections from ceilings. These may be one-way mirrors that are designed to allow one-way surveillance of the store. From one side, this surface looks like a mirror, but from the other side, the activities of the shoppers can be observed. Suggest how this type of mirror works.


Answer I think:

All light received by the observer is reflected by the glass. No light comes from inside.


PLEASE HELP ME WITH THIS QUESTION IF I AM WRONG.

 

[haruspex]

The passage of light is symmetric. The mirror cannot literally let light through in only one direction. Assume that it let's through some small fraction in each direction, reflecting the rest. Can you still explain it?