A plane mirror produces a magnification of

The magnification of a plane mirror is +1. This is because a plane mirror creates an image that is the same size as the object. The image is also virtual, upright, and located the same distance from the mirror as the object, but on the opposite side. The magnification is calculated by dividing the height of the image ($h_i$) by the height of the object ($h_o$): $\begin{aligned} & m=\frac{h_i}{h_o}=1 \\ & h_i=h_o \end{aligned}$