It's a familiar warning, but why does it have to be there in the first place?

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Why are objects in the side-view mirror closer than they appear?

Driver? Passenger? Never been in a car in your life? You probably still know the line: "Objects in mirror are closer than they appear." It appears in so many contexts it's nearly lost all meaning. Were it not for its continued presence on every right side-view mirror in North America, it may have been consigned to confusing-proverb status long ago.

In fact, the statement is a safety warning (or disclaimer, if you want to be cynical about it) intended to decrease driver misperceptions of the space to the immediate right of the car. It means exactly what it says: When a driver sees a car in the passenger-side mirror, that car is closer than the reflection would indicate.

Good thing to know, for sure. The passenger mirror distorts the driver's perception of an object's distance. But it's a safety trade-off, because the same mirror, and for the same reason, enhances the driver's perception of something arguably more essential to avoiding accidents.

It starts with the science of light and mirrors and creating images -- in this case, how a particular kind of mirror can distort size, and therefore distance.

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The Opposite Curve

Concave mirrors recede in the center and bulge at the edges, like a bowl turned right-side up. This type of mirror can make an object look either larger or smaller, and either upside-down or right-side up, depending on its distance from the mirror.

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On the Science Side: Light and Mirrors

It's easy to see that images in the passenger side-view mirror are smaller than they are in reality. All you need to do is check the mirror and then glance over your right shoulder. And this is why they seem farther away -- we judge distance by the relative size of objects. Our brains compare how big a car looks in the mirror with how big it is in real life, and the greater the discrepancy, the greater the perceived distance.

That's not the only thing going on here, though. In fact, the distance thing is a side effect.

To gloss over some basic physics: We can't actually see light until it hits something [source: Flinn]. When it bounces off an object and lands at our eye, we can see it -- as an image.

Light reacts in specific ways to an object's various surface traits (color, texture, shape, etc.), producing visual data our eyes can interpret to construct the image the light bounced off of.

When light reflects off a flat object, like an image in a flat mirror, it bounces off in the same direction at which it hit. The path of the light carrying the data back to our eyes is unaltered, and the image our eyes construct is in tune with reality.

Curve that mirror, though, and you alter the path the light travels to reach our eyes. Let's say you curve the mirror so that the center bulges toward your eye. This mirror is now convex, like a bowl turned upside-down, and like the side-view mirror on the passenger side.

When light hits a convex mirror, the curved surface changes the light's behavior. Close to the center, light bounces in a relatively unaltered path to our eyes; the farther outward the light hits, the farther outward it bounces. The result is that when light hits a convex mirror, the rays diverge, spreading out before reaching our eyes.

So, let's say light is bouncing off an image of a car in the convex side-view mirror ...

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On the Passenger Side: Size and Distance

If rays of light are carrying the image of a car in a mirror, and those rays travel outward before hitting your eye, the car that reaches your eye is not exactly the same as the car that reflected the light.

To be more specific, it's not in the exact same location. To our eye, the image carried in light reflected from a curved surface (actually any surface, curved or not) appears to be located where those light waves intersect. This is the focal point. Diverging light waves, however, would only intersect if they were to continue through the mirror, to the other side. This means the image of the car seems to be located behind the mirror, at a greater distance from your eye.

The images reflected in a convex mirror, then, look smaller than they are -- they're compressed. This is why convex mirrors are used on cars: They reflect more in a smaller space. In other words, a convex mirror has a wider field of view than a flat one, which can only reflect the area right in front of it. With a wider field of view, the driver has more information about the area to the right of the car.

This is the safety trade-off. A convex mirror sacrifices accurate distance perception for a wider field of view. And a wider field of view means a much smaller blind spot than you have on the driver's side of the car.

To avoid image distortion on the driver's side, U.S. regulations require driver's side mirrors to be flat [source: Taub]. Unfortunately, because flat mirrors have a very narrow field of view, there is a substantial area next to the car that they don't reflect. Elsewhere, such as in Europe, large blind spots can be avoided, because regulations allow both side-view mirrors to be convex [source: Taub]. Two wide-angle mirrors can cover a whole lot of (slightly distorted) space.

The current trade-off may not be the end of it, though. In May 2012, a Drexel University math professor patented a mirror that is slightly, calculatedly curved to reflect a wider field of view with less distortion [source: PHYS]. It's convex, yes; but objects in that mirror are about as close as they appear.

Whether this mirror is widely adopted and, if it is, whether it's required to carry the warning/disclaimer we've come to know and love, remains to be seen.

For more information on curved mirrors, driver safety, and related topics, check out the links on the next page.

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Lots More Information

Author's Note: Why are objects in the side-view mirror closer than they appear?

The math professor's mirror -- his name is Andrew Hicks, BA, MA, Ph.D., by the way -- is worth checking out. Data shows its field of view is roughly three times that of the driver's side mirrors we now use in the United States, and the image distortion it produces is practically nil. The mirror has received attention as a possible product for European markets [source: Drexel]. The inventor says it was inspired by a disco ball. You can read about the science behind it here.

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Sources

  • Flinn, Gallagher. "How Mirrors Work." HowStuffWorks. (Oct. 9, 2012) http://science.howstuffworks.com/innovation/everyday-innovations/mirror.htm
  • "Image Formation Revisited." The Physics Classroom. (Oct. 12, 2012) http://www.physicsclassroom.com/class/refrn/u14l5c.cfm
  • "Math professor's side mirror that eliminates 'blind spot' receives US patent." PHYS.org. June 7, 2012. (Oct. 9, 2012) http://phys.org/news/2012-06-math-professor-side-mirror-patent.html
  • "Reflection and Image Formation for Convex Mirrors." The Physics Classroom. (Oct. 9, 2012) http://www.physicsclassroom.com/class/refln/u13l4a.cfm
  • Safkan, Yasar. "Why does the passenger side window on my car state 'objects in mirror are closer than they appear?" PhysLink. (Oct. 9, 2012) http://www.physlink.com/education/askexperts/ae449.cfm
  • Taub, Eric A. "In Defense of Convex Driver's-Side Mirrors." Wheels – The New York Times. Dec. 30, 2010. (Oct. 9, 2012) http://wheels.blogs.nytimes.com/2010/12/30/in-defense-of-convex-drivers-side-mirrors/