Convex mirror

Also called Diverging mirror

A mirror curved so that its reflecting surface bulges outward, spreading parallel rays so they appear to come from a focal point behind it. Its focal length is negative and every image it forms is virtual, upright and reduced.

A diverging mirror, written by the CED as "convex (diverging) mirror", which places the focal point behind the mirror in its own words.

Negative focal length, because behind the mirror is the region light never reaches, and the sign convention makes that the negative side. Its magnitude is $R/2$ as for any spherical mirror, so $f = -R/2$ .

No cases at all. For any real object at any distance the image is virtual, upright and reduced, sitting behind the mirror. One line proves it: with $f$ negative,

\frac{1}{s_i} = \frac{1}{f} - \frac{1}{s_o}

is negative minus positive, so $s_i$ is negative for every positive $s_o$ . The same expression forces $|s_i| < s_o$ , so the magnification magnitude never reaches 1.

That is exactly what a security mirror and a car passenger mirror are for. Shrinking everything buys a wide field of view, which is also why such mirrors carry the warning that objects are closer than they appear.

What makes it the opposite of a [concave mirror](/glossary/concave-mirror): a concave mirror can gather light to a real focus and project a real image, and it changes behaviour as the object crosses $f$ . A convex mirror does neither. It behaves instead like a diverging lens, since diverging is diverging whichever way the light goes.

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