Image Formation by Plane Mirrors

Both concave and convex spherical mirrors asymptote to plane mirrors in the limit in which their radii of curvature $R$ tend to infinity. In other words, a plane mirror can be treated as either a concave or a convex mirror for which $R\rightarrow \infty$. Now, if $R\rightarrow \infty$, then $f=\pm R/2\rightarrow\infty$, so $1/f\rightarrow 0$, and Eq. (358) yields

$$\frac{1}{p} + \frac{1}{q} = \frac{1}{f} = 0,$$ (359)

or

$$q = -p.$$ (360)

Thus, for a plane mirror the image is virtual, and is located as far behind the mirror as the object is in front of the mirror. According to Eq. (352), the magnification of the image is given by

$$M = -\frac{q}{p} = 1.$$ (361)

Clearly, the image is upright, since $M>0$, and is the same size as the object, since $\vert M\vert=1$. However, an image seen in a plane mirror does differ from the original object in one important respect: i.e., left and right are swapped over. In other words, a right-hand looks like a left-hand in a plane mirror, and vice versa.