angle of reflection
Summary
The angle of reflection is the angle between a reflected ray and the normal, the line perpendicular to the surface at the point of contact. By the law of reflection it equals the angle of incidence, and both are measured from the normal, not from the surface.
Detail
Meaning: When light hits a smooth boundary, part of it bounces back. The angle of reflection is the angle between the reflected ray and the normal, the line perpendicular to the surface at the point of contact. The incident ray, reflected ray and normal lie in one plane. A rough surface scatters light (diffuse reflection) because its local normals point different ways, yet each ray still reflects with equal angles about its own local normal.
Equation: θr = θi, where θr is the angle of reflection and θi is the angle of incidence, both measured from the normal, in radians.
On the MCAT: Expect plane-mirror ray diagrams, spherical mirrors (the normal lies along the radius), or a boundary where light both reflects and refracts. The reflected angle equals the incident angle whatever the refractive indices, while the refracted angle follows n₁ sin θ₁ = n₂ sin θ₂. Rotating a mirror by φ turns the reflected ray by 2φ.
Common confusion: Students measure from the surface instead of the normal. A ray striking a mirror at 30° to the surface has an incident angle of 60° from the normal, so the reflected angle is also 60°.
Sources
- Fundamentals of Physics
- University Physics
- Physics (Giancoli)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.