Making a one-way mirror may seem complex. After all, you probably have to do something to one side of a mirror to make it transparent. Either that, or you have to do something to one side of a window to make it look like a mirror. In truth, the process is much simpler. The key to a one-way mirror is in the lighting.
Place the thinly silvered mirror between two rooms. The silver on the mirror will reflect a small amount of light, but most of the light will pass through the mirror.
Adjust the lighting so that one of the adjoining rooms is very dim and the other is very bright.
Look through the mirror while standing in the bright room. In this room, a lot of light is directed at the mirror. Therefore, even the small amount that is reflected is enough to make the surface look like a mirror.
Look through the mirror while standing in the dim room. In this room, very little light is directed at the mirror. However, a lot of light from the other room is passing through the mirror into the dim room. Therefore, you will be able to see through the mirror.