Image Formation By A Plane Mirror:

The formation of image by a plane mirror is due to the fact that says traveling
from on object to our eye change their direction after reflection so that they
appear to come from point other than those from which they really started.

Explanation:

Suppose light rays coming from luminous object 'o' are reflected by the plane mirror 'M' and
enter our eye.The line which joins the image 'I' and the object 'o' makes an angle
of 90' with the surface of the mirror 'M' from the geometrical construction, the
distance 'OM'and 'IM' are equal, therefore we feel that the light rays come from 'I'
but in fact they come from 'o' and are reflected by the mirror.At the mirror surface,
we find that the angle of incidence is equal to the angle of reflection.

Characteristics Of The Images Formed By A Plane Mirror:

Images are laterally inverted (Right side appear as left).
Images are of same size as the object.
Images are virtual (Can't be obtained on a screen).