A mirror with a flat reflecting surface is known as a plane mirror.
For a light beam incident on a plane mirror, the angle of incidence and the angle of reflection are equal. The angle of incidence is the angle formed by the incident ray and normal (a line perpendicular to the mirror surface) and the angle of reflection is the angle formed by the reflected ray and normal.
Image formed by a plane mirror:
The image formed by a plane mirror is virtual, it is formed by tracing back the rays. The position of the image is behind the mirror.
The image formed is erect, that is right-side up.
It is the same size as the object.
Also, the distance between the object and the mirror is equal to the distance between the image and the mirror.
Hence, the image formed by the plane mirror is virtual, erect, and of the same size.