angle of incidence

The angle of incidence is the angle between an incoming light ray and the normal at the point where it strikes a mirror.

• It's measured from the normal, not from the mirror's surface — a ray striking almost flat along the surface has a large angle of incidence, close to 90°, while one striking straight into the mirror has an angle of incidence close to 0°.
• The first law of reflection ties this angle directly to its partner, the angle of reflection: however large or small the angle of incidence is, the angle of reflection always matches it exactly.
• Changing the angle of incidence is exactly how spherical mirrors turn a straight beam of parallel light into a converging or diverging cone, since each incoming ray meets the curved surface — and its own local normal — at a slightly different angle.
• This isn't just an abstract rule: it's the actual reasoning tool used to predict exactly where a reflected beam will go, given only where the light comes from and the shape of the mirror.