Why Are Convex Mirrors Used As Rearview Mirrors?

by Amina Elahi

Anyone who has sat in the driver's or passenger's seat in a car has probably noticed the tiny wording, "Objects in mirror are closer than they appear," that runs along the bottom of the passenger-side rearview mirror. Some may question the logic of making objects seem like they are farther away than they actually are, but the reason is that using a convex mirror this way gives the driver the best range of vision. In contrast, the wide windshield mirror is flat because it is meant to show what is directly behind the car, not far off to the sides.

What is a Convex Mirror?

A convex mirror is one that curves outward; that is, it curves toward you. Convex mirrors are known as diverging mirrors because when light reflects off of them, the rays move in opposite directions. In contrast concave mirrors, which curve inward, cause reflected rays to converge after reflection. Another unique aspect of convex mirrors is that they create virtual images. When the light rays are reflected, they can be extended to meet the image position but do not naturally go through it on their own.

How They Work

Convex mirrors are used to see behind cars because the images they produce are virtual but still upright. That is, the image you see in your rearview mirror may be scaled differently, but it will not be flipped upside down. Convex mirrors work two ways. First, they can reflect parallel rays of light in such a way that they scatter in all directions. On the other hand, they can take light coming from various directions and reflect them so that they are parallel. It is this second method that is used in cars.

Benefits

For drivers, the benefit of having convex rearview mirrors is simple. Convex mirrors allow drivers to see more because they provide a greater field of view by scaling down the objects they reflect. To compare, flat mirrors show objects at accurate distances and sizes, and concave mirrors, while able to show real images, often invert images as well. Therefore, convex mirrors provide the best upright range of vision for drivers.

Detriments

The trade-off in using convex mirrors for car rearview mirrors is that they make it seem like those objects are at a greater distance than is true. This is why the warning "Objects in mirror are closer than they appear" is necessary on car mirrors.

Other Uses

Use a convex mirror to see who is coming up the road beyond a blind turn.

In addition to rearview mirrors, convex mirrors are used in many familiar settings. Store owners often place them high in far corners in order to keep an eye on distant aisles. The virtual, scaled image such mirrors reflect allows the shopkeeper to see more of what is going on in the store. Additionally, convex mirrors can be used near blind turns to give drivers a glimpse at what may be hiding behind the corner. These are often seen in parking structures. In both of these cases, as in the car example, convex mirrors are used for safety and security.

About the Author

Amina Elahi is a senior at Northwestern University. She has written for Popular Science, Cafe Magazine and the Evanston Review. A writer since high school, Elahi began contributing to professional publications three years ago. Additionally, Elahi has edited two publications at Northwestern--one website and one magazine.

Photo Credits

  • photo_camera classic rearview mirror image by Andrew Kazmierski from Fotolia.com