When a ray of light strikes a mirror perpendicular, what law does it states

According to the law of reflection, when a ray of light strikes a surface, the angle of this ray (incident ray) with respect to the normal of the surface is equal to the angle that forms the reflected ray with the same normal of the surface.

In other words:

Both the incident ray and the reflected ray and the normal to the surface are in the same plane, in addition, the angle of the incident ray is equal to the angle of the reflected ray.

In this context, when a ray strikes the surface perpendicular to it, both the incident angle and the reflected angle will be zero, since the normal to the surface is also perpendicular to it.