Rear-View Mirrors

Stupid Question ™
Dec. 13, 2001
By John Ruch
© 2001

Q: You know that little lever underneath your rear-view mirror that makes the reflection darker? How does that work?
—B. Jones


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.

Objects In Mirror Closer Than They Appear

Stupid Question ™
Sept. 23, 1999
By John Ruch
© 1999

Q: Why are objects in a car mirror closer than they appear?
—R.P.


A: Mirrors that carry the familiar warning, “Objects in mirror are closer than they appear” are not regular flat mirrors. They are slightly convex, meaning they bulge outward like the side of a ball.

Convex mirrors are useful because their large surface area allows them to reflect light from a much bigger area than a flat mirror of the same size. You’ve probably seen large, round convex mirrors in convenience stores, where they’re used to observe the entire store from one point.

According to General Motors spokesperson Terry Rhadigan, convex mirrors are now ubiquitous on the passenger side of cars because they help eliminate blind spots.

Flat mirrors are fine for the driver’s side and rear-view, because they’re close enough that the driver can adjust his or her head to see a wide range of angles. But the passenger-side mirror is far away, and, if flat, would offer only a very limited field of vision. Making it convex gives you a field of vision about 30 percent bigger, so changing lanes is much safer.

However, the convex mirror also creates what Ohio State University physics professor Linn Van Woerkom calls “a classic geometric optics problem”: this useful wide-angle reflection also results in a distorted image.

In a flat mirror, light rays bounce off at regular 90-degree angles, producing a near-perfect duplicate of the reflected object. The reflected object will look as big as in real life, and as far “in” in the mirror as it is away from the mirror.

But convex mirrors gather light rays around the curved surface and reflect them at varying angles. One result is that the image is compressed around the most bulging part, and stretched out near the mirror’s edges.

This compression occurs in such a way that the image can never be as large as (or larger than) the object that’s being reflected. A car, for example, seen in a convex mirror will look smaller than it really is.

When we’re very young, we learn through experience that when a familiar object looks relatively smaller, it is usually farther away—even in flat mirrors. The smaller it looks, the farther away it is (and vice versa).

Since convex mirrors shrink images, they can fool us. If you’re not used to a convex car mirror (which is so slightly convex it looks flat), you may think the slowpoke car you want to pass is farther away than it really is, cut somebody off, and cause a wreck.

Hence, the warning that objects in the mirror are closer than they appear.
Or hence the warning in the US, anyway. A Ford spokesperson tells me that many cars in Europe use convex mirrors on both sides of the car for extra safety, and none of them carry the warning at all.

Perhaps the product-liability warning is just a reflection on our litigious society.