

Let an object at M be placed in front of a plane mirror AB at a distance r (as shown in the given figure).

A circle is drawn from the centre (M) such that it just touches the plane mirror at point M1. According to Huygens’ Principle, AB is the wave front of incident light.
If the mirror is absent, then a similar wave front A1B1 (as AB) would form behind M at distance r (as shown in the given figure).

A1B1 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).
