A one-way mirror is a reciprocal mirror which is also called a two-way mirror. One-way mirror observation is obtained when one side of the mirror is dark and the other side looks bright.
A one-way mirror is basically used in a brightly lit area with a dark area on its other side and it acts as a normal mirror. We can see our reflection on the brightly lit side of the one-way mirror. You can also know more about smart mirror display technique by clicking at https://mangodisplay.com/smart-mirror/
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The one-way mirror looks like a transparent window when you see it from its dark side. The light is reflected back from the mirror itself in a bright room which is much greater than the light comes from the darkroom.
The small amount of light passes from the darkroom to a bright room. Equally, the light reflected back to the dark side of the room to overcome the light comes from the bright side of the room.
One way a mirror is used for one-way observation. the observation is done in dark sided rooms. these rooms are used for experimental psychology research and execution chambers.
The advantage of building your own smart mirror is the software and modules that other people have already built. Many people have built hundreds of different modules used for smart mirrors.