Two way mirrors work because they allow some light to pass through and reflect the rest of the light back to the observer. In order for a two way mirror to work one side of the mirror must be kept brightly lit and the other side has to be very dimly lit.
On the dimly lit side of the glass very little light can make it through the glass so people on the other side do not have enough light waves to see. There also are not enough light waves relfecting for people on the dimly lit side to see their own reflections.
On the brightly lit side since some of the light is reflected, the people on this side can see their own reflection. People on the other side can see the light that was transmitted through the glass so they can also see what is happening on the brightly lit side of the mirror. It just looks like they are looking through dark colored glass.
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