When drivers traveling at night encounter a bright light from the vehicle behind them shining in their eyes, many will flip a tab on the underside of the rearview mirror to deflect the light up toward the ceiling of the car. The silvered surface of the mirror reflects approximately 85–90% of the incident light on the mirror, which is now directed toward the ceiling. The remaining 10–15% of the light is reflected by the front of the glass on the mirror. The mirror is wedge-shaped, thicker at the top than at the bottom. Thus the front surface is angled downward to allow the much smaller amount of light to be reflected into the driver’s eyes.
The word “AMBULANCE” is printed backwards on these emergency vehicles so that drivers will see the word correctly in their rearview mirrors.