When a ray of light strikes a surface, it can be reflected, refracted, or absorbed. When light is reflected, the angle between the incident ray (the ray of light before it hits the surface) and the reflected ray (the ray of light after it hits the surface) is equal to the angle between the incident ray and the normal to the surface (a line perpendicular to the surface at the point of incidence).
In the case of a plane mirror, the surface is flat, so the normal to the surface is perpendicular to the mirror. When a ray of light strikes a plane mirror normally, it means that the ray of light is traveling parallel to the normal to the surface. In this case, the angle between the incident ray and the normal to the surface is zero. Since the angle between the incident ray and the reflected ray is equal to the angle between the incident ray and the normal to the surface, the angle between the incident and reflected rays is also zero.
In other words, when a ray of light strikes a plane mirror normally, it is reflected directly back on itself.
