Objects in Mirror are larger than they appear
We have all seen this message printed on the passenger side rear view mirror of our vehicles. At one point or another everyone has wondered why the objects are closer than they appear?, others know why it is, while some just don't care, but above all we have learned to live with it as a part of our daily lives. "Why don't they just put: This mirror is broken, do not use" (Gallagher). The reason this mirror contains this message is that it's a convex mirror, not a flat mirror like the driver side rear view mirror.
So how does a convex mirror function then?
When the rays from a object hit a convex mirror they reflect off the surface of the mirror. If these reflected rays were to be extended behind the mirror, they would combine at a single point to form a image. This image is formed where the rays emanating from the object appear to intersect behind the mirror. Notice, the image you see is formed behind the mirror not in front of it. Convex mirrors form images that are smaller than the object itself.
Now, we can say that the image seen in the passenger side rear view mirror is: smaller than it normally would be, is behind the mirror, and that the farther away a object is the smaller it appears to be. Taking all this into account we can conclude that objects in the passenger side rear view mirrors are closer than they appear to be.
The image formed by the mirror is smaller than it would be in a flat mirror making the statement about the mirror true, but the mirror doesn't stop there. It also puts the image behind the mirror which according to the laws of depth perception makes the image even smaller. This is why it is a good idea to look back to see how close that car is, even after you look in your mirror.