[farmall]

A prismatic rear-view mirror — sometimes called a "day/night mirror" — can be tilted to reduce the brightness and glare of lights, mostly for headlights shining directly on the eye level 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 reflectorized rear surface is tilted out of line with the driver's view. This view is actually a reflection off the non-reflectorized front surface. Since the non-reflectorized front surface allows most of the light to go through, only a small amount of light is reflected into the driver's eyes


From Wiki.