A rearview mirror with a night position is actually two reflective surfaces.
One is a shiny mirror that is about 90 percent reflective. Over it is a wedge of glass that is only about 4 percent reflective.
The wedge is very thin and angled only a few degrees.
In the daytime, the mirror is angled so that the shiny side reflects the outside light.
The less shiny side is pointing somewhere at the driver’s pants leg, but all the driver sees is the reflection from the shiny back surface of the glass, through the wedge.
There is also a 4 percent reflection from the wedge, but the driver doesn’t see that, because so much light is coming in from outside.
At night, a lever switches the mirror to a different angle, and the process is reversed.
Now the shiny side is reflecting nothing but dark roof of the inside of the car, where there is very little light.
But when the bright beams of a car hit the less reflective glass wedge, the light can be seen without blinding the driver.