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.

Explanation

Let an object at O be placed in front of 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 Huygen's principle , XY is the wavefront of incident light.
If the mirror is absent, then a similar wavefront X'Y' (as XY) would from, behind O' at distance r (as shown in the given figoure).

X'Y' 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).