You have learnt in the text how Huygens' principle leads to the laws of reflection and refraction. Use the same principle to deduce directly that a point object placed in front of a plane mirror produces a virtual image whose distance from the mirror is equal to the object distance from the mirror.

(N/A) Let an object at $O$ be placed in front of a plane mirror at a distance $r$ (as shown in the figure).
$A$ spherical wavefront is emitted from the point object $O$. The part of the wavefront $XY$ reaches the plane mirror at point $O'$.
According to Huygens' Principle,each point on the wavefront acts as a source of secondary wavelets. When the wavefront $XY$ strikes the mirror,the mirror reflects these wavelets.
If the mirror were absent,the wavefront would continue to propagate and form a similar wavefront $X'Y'$ at a distance $r$ behind the mirror.
Since the mirror reflects the light,the secondary wavelets appear to originate from a point $I$ behind the mirror,which is the virtual image of $O$. By symmetry,the distance of this virtual image $I$ from the mirror is equal to the distance $r$ of the object $O$ from the mirror.