Stupid Question ™
Dec. 13, 2001
By John Ruch
Q: You know that little lever underneath your rear-view mirror that makes the reflection darker? How does that work?
A: Flipping that little lever actually makes the mirror tilt up or down, changing the angle at which light rays strike it. Due to the mirror’s unusual construction, this slight change in angle results in a huge difference in the amount of reflected light.
We’re most familiar with flat mirrors that consist of a reflective metallic backing covered with a flat sheet of glass.
But the rear-view mirror’s metallic backing (typically aluminum) is covered with a wedge-shaped chunk of glass—shaped like an upside-down right triangle—that is thickest at the top and thinnest at the bottom. Known as a prismatic mirror, this design means the glass surface of the mirror is angled inward at the bottom. (There are various prismatic mirror designs in optics; this specific design is what car makers always mean by the term.)
For normal driving, you tilt the mirror down to your eye level, and it bounces light rays from behind you as a standard mirror would. When you flip the lever for night driving, the mirror tilts and angles up at the car’s ceiling, which is where it reflects the majority of headlight glare. (Next time some idiot high-beams you, look up there and you’ll see the light reflected onto the ceiling.)
However, some of the headlight-beam light still gets to your eyes—enough to see where other cars are—because the glass surface of the mirror is still angled down toward you. The light you see doesn’t come from the actual mirror—the reflective metal backing—but only from the outer surface of the glass. It’s similar to looking at a reflection in a windowpane from an angle.
The glass surface reflects under 10 percent of incoming light to your eyes, and the image you see is thus about 20 to 25 times dimmer than the actual light source. This kills most headlight glare.
However, there are problems with the prismatic mirror. If the light is bright enough, as it often is with new-fangled halogen headlights, the mirror won’t kill all the glare. When used in the daytime to cut sun glare, it can be too dim to reflect cars that aren’t using headlights. And especially in cheap models, the design can cause ghost images—double reflections from the mirror and the angled glass surface.
The new trend is to replace prismatic mirrors with ones that automatically dim themselves. They use either a liquid-crystal gel in the glass that darkens automatically when exposed to strong light, or a layer of photoelectric cells that darken proportionately when sensors report an imbalance between ambient light and the light actually hitting the mirror. These designs cut more glare, sacrifice less visibility and can be used in side mirrors as well.