The law of reflection states that the angle of incidence equals the angle of reflection. define the angle of incidence in terms of a line normal to the surface.

Answer : According to the definition of law of refelection "It is the principle in which, when a ray of light or radar or pulse, is shined on a smooth surface it is reflected back from the smooth surface and the angle of reflection will be equal to the angle of incidence, also the incident ray, the reflected ray, and the normal to the surface at the point of incidence all will lie in the same plane."

So we can define angle of incidence as - The angle which is formed between the incident ray and the smooth surface of substrate is called angle of incidence and the incident ray will be always intersecting at the point of normal with the reflected ray. So the normal is an imaginary line which is perpendicular to the smooth surface of the substrate which bisects the ray of incidence and forms the angle of incidence also it bisects the ray of reflection and forms angle of reflection.

Iwould you figure it out but you don't have a picture or in your question as to what the physical property you're looking for is