Let an object at $O$ be placed in front of a plane mirror $MO'$ at a distance $r$ (as shown in the given figure).

A circle is drawn from the centre $(O)$ such that it just touches the plane mirror at point $O'$. According to Huygens' Principle, $XY$ is the wavefront of incident light. If the mirror is absent, then a similar wavefront $X'Y'$ (as $XY$ ) would form behind $O'$ at distance $r$ (as shown in the given figure).

$X^{\prime} Y^{\prime}$ can be considered as a Virtual reflected ray for the plane mirror. Hence. a point object placed in front of the plane mirror produces a virtual image whose distance from the mirror is equal to the object distance $(r)$.