Anyone who has glanced in a vehicle’s sideview mirror should be familiar with this message: “Objects in mirror are closer than they appear.” But did you ever wonder why that’s the case? The answer comes down to physics.

Here’s a brief rundown on the science behind sideview mirrors, along with tips on how to use them properly to help stay safe on the road.

How Convex Mirrors Work

You may have noticed that the surface of your driver’s side mirror is flat, while the surface of your sideview mirror on the passenger side is a convex shape. A convex mirror curves slightly like the outside of a large bowl, according to the University of Illinois at Urbana-Champaign (UIUC). You may have also noticed that due to its convex shape, the image in the mirror looks farther away than it truly is, notes. Here’s why.

An image is created in a mirror when light hits the surface. A convex mirror bends light as it is reflected. The farther away a point is from the center, the more the light is bent, according to the UIUC. As a result, the image formed in a convex mirror is smaller than an image in a flat mirror, the university explains.

Because the images are smaller, more fits on the surface of the mirror, UIUC says. For drivers, a convex mirror offers a wider field of vision.

Properly Using Sideview Mirrors

Sideview mirrors are designed to help drivers get a clear and reasonably unobstructed view of the rear area around the vehicle, according to Federal Motor Vehicle Safety Standards. These standards were established to regulate the mirror’s design to help give drivers a reasonably unobstructed view and prevent accidents.

Following these mirror tips may help you stay safer on the road:

  • Check and adjust your mirrors when you get in your car, especially after someone else has been driving it, suggests YourMechanic.
  • When adjusting your mirrors, be sure you’re positioned correctly in the seat and can easily reach the pedals and wheel, YourMechanic notes. Then, adjust the side mirrors outward until you can no longer see your own car, says Car and Driver.
  • Keep your mirrors clean and in good condition to get the clearest views possible, according to the International Carwash Association.

A car’s mirrors help drivers see their surroundings. Knowing how these mirrors work can help you drive more safely.