As you likely discovered in your research and laboratory investigation, the angle of incidence equals the angle of reflection. You can use this relationship in problem solving as long as you know the terms used and the angles referenced. Study the image below paying close attention to the angles and how they are referenced.
Notice that the angles of incidence and reflection are both the angle between the ray and a line that is perpendicular to the surface. In physics, we call this the normal. Just like the normal force is perpendicular to the surface an object is sitting on, the normal line is perpendicular to the surface where the light ray hits.
Question
What happens to a light ray that comes into a surface at perpendicular angle to the surface?
A ray that comes in perpendicularly to the surface will reflect perpendicularly to the surface. In other words, the incident angle and reflected angle are both zero.
