Objects in mirror are closer than they appear

Most North Americans are familiar with the warning “Objects in mirror are closer than they appear.” If you’ve ever wondered why objects in your car mirror are closer than they appear, or why the passenger side mirror carries this warning but not the driver’s side – wonder no more! The thrilling answer is here!

I thought about this today when my passenger side mirror flew off. The wing mirror assembly is still there, just the glass part is gone. Just gone. Literally in a flash. The warning is still visible, interestingly enough. Also interestingly, the dealership will not replace just the glass. The entire wing assembly would have to be replaced, costing in the neighborhood of $400. US DOLLARS, guys. The local glass place apparently replaces these things all the time for around $30.

Anyway. My long-standing assumption is that the passenger side mirror carries this warning because the driver is further away from the mirror on that side, and the reflection would therefore be correspondingly smaller. This sounds good, because if you stand a foot from a mirror and look into it, your image reflects as if it were a person standing two feet away, right? (Yes, correct.) But that’s for a straight-on reflection. Your passenger mirror doesn’t really care how far away you are from the image it’s reflecting, because it’s not reflecting you; it’s reflecting stuff behind and beside your car.

Apparently, driver side mirrors must, in the US and Canada, reflect 1:1 perspective. That answers half the question (why the driver side mirror doesn’t have the same message.) In the rest of the world, driver side mirrors can be convex, planar or aspheric or a combination (with a line delineating where the change in magnification is). Thanks Wikipedia! The rest of the answer also comes from there, if you were curious. As it turns out, thrillingly for me, part of the answer does have to do with how far away the driver sits, because the visible area able to be viewed is too limited with a flat mirror, so it must be distorted in order to get some scope. You know what concave and convex are, but I’ll briefly walk through those, anyway. Concave is when the middle of a surface bows in. Convex is when the surface bows out. When a mirror is made convex, the objects in the mirror appear smaller. That is why the mirror carries the warning, because it is a convex mirror.

An interesting tidbit, also from Wikipedia: European wing mirrors must meet a regulation that they swing away when struck, as by a pedestrian. Good on them.

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