Anti-glare
A prismatic rear-view mirror—sometimes called a "day/night mirror"—can be tilted to reduce the brightness and glare of lights, mostly for high-beam headlights of vehicles behind which would otherwise be reflected directly into the driver's eyes at night. This type of mirror is made of a piece of glass that is wedge-shaped in cross section—its front and rear surfaces are not parallel.
On manual tilt versions, a tab is used to adjust the mirror between "day" and "night" positions. In the day view position, the front surface is tilted and the reflective back side gives a strong reflection. When the mirror is moved to the night view position, its reflecting rear surface is tilted out of line with the driver's view. This view is actually a reflection off the low-reflection front surface; only a much-reduced amount of light is reflected into the driver's eyes.
"Manual tilt" day/night mirrors first began appearing in the 1930s and became standard equipment on most passenger cars and trucks by the early 1970s.
Read more about this topic: Rear-view Mirror