Buzz
It's a well-known phrase, but why is it necessary to include it in the first place?
Walter Giordani/Workbook Stock/Getty ImagesWhether you're the driver, a passenger, or someone who has never set foot in a car, you're likely familiar with the phrase: "Objects in mirror are closer than they appear." This phrase has appeared in so many places that its meaning has almost faded, but if it weren't for its consistent placement on every passenger-side mirror in North America, it might have been forgotten as just a confusing proverb by now.
In fact, it's a safety caution (or perhaps a disclaimer, if you're feeling skeptical) designed to reduce the chances of drivers misjudging the space immediately beside the car. It literally means what it says: When a driver observes a vehicle in the passenger-side mirror, that vehicle is actually much closer than the mirror image would suggest.
It's certainly valuable to understand that the passenger side mirror can distort the driver's perception of how far an object is. However, it's a necessary safety trade-off. The same mirror, for the same reason, also helps enhance the driver's awareness of objects that are crucial to preventing accidents.
It all begins with the principles of light and mirrors and how images are formed—specifically, how a certain type of mirror can create distortions in size, and thus distance.
On the Science Side: Light and Mirrors
It's quite evident that images in the passenger side mirror appear smaller than their true size. All you need to do is check the mirror and then quickly look over your right shoulder. This is why the objects seem farther away—the size comparison in our brains helps us judge distance. The greater the size discrepancy, the more distant an object seems.
However, the distance perception is just one aspect of what's happening here. In fact, it’s a secondary effect of a larger phenomenon.
To simplify a bit of basic physics: Light itself is invisible until it strikes something. Once it bounces off an object and enters our eyes, we are able to perceive it as an image. [source: Flinn].
Light interacts with the surface properties of objects—such as color, texture, and shape—causing it to create visual information that our eyes can interpret to form an image based on the object the light has reflected off of.
When light reflects off a flat surface, like in a regular mirror, it bounces back in the same direction it came from. The path of the light remains unchanged, so the image that we perceive aligns with the true object.
But if you curve the mirror, you change the path the light follows to reach our eyes. For instance, if you make the center of the mirror bulge toward you, it becomes a convex mirror—like an upside-down bowl, similar to the passenger-side side-view mirror.
When light strikes a convex mirror, the curve alters the way the light behaves. Close to the center, light reflects relatively directly back to our eyes; however, the farther from the center the light strikes, the more outwardly it bounces. The result is that light rays diverge when they hit a convex mirror, spreading out before they reach our eyes.
Imagine light bouncing off the reflection of a car in a convex side-view mirror...
Concave mirrors curve inward at the center and bulge outward along the edges, much like a bowl placed upside down. This mirror type can alter the appearance of an object, making it appear either larger or smaller, and can flip it upside down or keep it upright, depending on how far the object is from the mirror.
On the Passenger Side: Size and Distance
When rays of light carry the image of a car in a mirror, and these rays spread out before reaching your eyes, the car you perceive is slightly different from the one reflecting the light.
To clarify, the car's image isn't exactly where it seems to be. In reality, the image seen in light that reflects off a curved surface (or any surface, for that matter) appears to be situated at the point where the reflected light rays intersect. This point is known as the focal point. However, diverging light rays would only meet if they continued behind the mirror, at a greater distance from your eye, making the car's image appear further behind the mirror than it actually is.
Reflections in a convex mirror appear smaller than the actual objects—they’re compacted. This is why convex mirrors are commonly used in vehicles: they reflect a broader area in a more confined space. Put simply, a convex mirror provides a wider field of view compared to a flat one, which only reflects what’s directly in front of it. With a wider field of view, the driver gains more awareness of the space to the right of the car.
This is the safety trade-off. A convex mirror exchanges precise distance perception for a broader field of view. And that wider field of view results in a considerably smaller blind spot than what is found on the driver's side mirror.
To prevent distortion on the driver's side, U.S. regulations mandate that the driver's side mirrors remain flat [source: Taub]. Unfortunately, flat mirrors have a narrow field of view, leaving a significant portion of space beside the car unreflected. However, in regions like Europe, large blind spots are less of an issue because regulations allow both side mirrors to be convex [source: Taub]. Two wide-angle mirrors can cover a larger (slightly distorted) area.
But the current trade-off might not be the final solution. In May 2012, a Drexel University math professor patented a mirror with a slight, carefully calculated curve designed to reflect a wider field of view with less distortion [source: PHYS]. It’s convex, yes, but the objects in this mirror appear almost as close as they really are.
Whether this mirror will be widely adopted, and if so, whether it will require the familiar warning label we’ve come to expect, remains uncertain.
For additional details on curved mirrors, driver safety, and related subjects, be sure to explore the links provided below.
