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Reflection Off A Convex (diverging) Mirror

rays traced for diverging mirror

This simulation shows the results of light striking a convex mirror which is also known as a diverging mirror. Notice that the light comes in parallel to principal axis of the mirror (not shown). As a result, the light gets reflected as though it came from the focal point of the mirror. The focal point is, by definition, where the light rays seem to cross after coming in parallel to the principal axis. Since they only "seem" to cross, it often called a virtual focus instead of a focal point. So for this mirror the focal point is where the five light "particles" seem to have overlapped behind the mirror. See the diagram below for a better drawing of where the focal point and principal axis are.
This "imaginary" overlap would occur whether light is considered to be a particle or a wave.

focal point

The following diagram shows the rules for the known behavior of light rays striking a Convex (diverging) mirror:

reflection rules

The following rules apply to light rays striking a convex (diverging) mirror:


Diverging mirrors are used as security mirrors in stores because they allow you to see a wide angle when you look into one.  This is a result of the image being made smaller, which allows more "space" in the mirror for a wider view.  They are often used on side view mirrors for cars because of their wide angle view.  The problem is that we tend to associate image size with how close an object is.  When you look into your side view mirror and see the small image of a car, you might assume it is a long way away, when in fact it might be right next to you.  For this reason they carry the warning "Objects in mirror are closer than they appear." 
(Don't go by the Meatloaf song, it is backwards!)

Far Side Cartoon

Thanks to Garry Larson's Far Side for the correct and humorous physics.