law of reflection

Meaning: When a wave such as light strikes a smooth boundary, the reflected ray leaves at an angle equal to the angle at which the incident ray arrived. Both angles are measured from the normal, the line perpendicular to the surface at the point of contact. The incident ray, reflected ray and normal all lie in the same plane. On a rough surface the law still holds at each tiny patch, but the normals point in many directions, so the light scatters (diffuse reflection).

Equation: θᵢ = θᵣ, where θᵢ is the angle of incidence and θᵣ is the angle of reflection, each in degrees or radians (the SI unit is the radian), measured from the normal.

On the MCAT: Passages ask you to trace a ray off one or more plane mirrors, often with a ray diagram, and find the final direction. Expect questions on the image in a plane mirror (virtual, upright, same size, as far behind the mirror as the object is in front) and on curved mirrors, where the law applies at each point and explains why a ray parallel to the axis passes through the focal point of a concave mirror. Reflection also applies to sound and other waves.

Common confusion: Students measure the angle from the mirror surface rather than the normal. If a ray makes 30° with the surface, the angle of incidence is 60°. Also, reflection does not change the frequency or the speed of the wave, because the wave stays in the same medium. Refraction, which does change speed, is a separate law (Snell's law).