Devices familiar to students are always good examples to illustrate physics concepts. I have found the right-side mirrors of cars to be one of these illustrative devices, especially because of the mysterious label printed on them: Warning! Objects in mirror are closer than they appear. Finding the physical reason for the label is a very interesting exercise that may be used to review or introduce some optical concepts.

See, for instance, E. Hecht and A. Zajac, Optics, 3rd ed. (Addison-Wesley, Reading, MA, 1998).
A.E. Siegman, Lasers (University Science Books, Sausalito, CA, 1986), p. 622.
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