Reflection
Also called Law of reflection
The bouncing of light back from a surface. The angle of incidence equals the angle of reflection, with both angles measured from the normal, the line drawn perpendicular to the surface at the point where the ray strikes.
The law is one line, and the whole difficulty is in where the angles are measured from:
From the normal, never from the surface. A ray striking a mirror at 20 degrees to the glass makes 70 degrees with the normal, and 70 is the number the law uses. Nearly every wrong answer on reflection is a complementary angle.
The AP Physics 2 CED lists as the relevant equation for Topic 13.1, and it is not among the fifteen entries in the Waves, Sound, and Optics group of the equation sheet. This one you know rather than look up.
Specular against diffuse. The distinction is not that one obeys the law and the other does not. Both do, at every point. What differs is the surface normal. On a smooth surface the normal has about the same direction everywhere the light lands, so a parallel bundle stays parallel and you see an image. On a rough surface the normal varies across the illuminated area, so each ray reflects to its own angle and the bundle scatters. That is why you read a printed page by diffuse reflection off paper but see your face in a window.
Some reflection happens at every boundary between media, including a fully transparent one, which is what makes thin-film interference possible. Past the critical angle it becomes complete: see total internal reflection. Topic 13.1 has the CED framing.