The angle of incidence equals the angle of reflection. Both angles are measured from the normal — a line perpendicular to the surface at the point where the ray strikes.
An imaginary line perpendicular (90°) to the reflecting surface at the point of incidence. All reflection angles are measured from the normal, never from the surface.
Parallel incident rays all reflect at the same angle. The virtual image appears as far behind the mirror as the object is in front, upright and the same size.
Rays parallel to the principal axis converge at the focal point F after reflection. Focal length f = R/2 where R is the radius of curvature. Used in torches, reflecting telescopes, and satellite dishes.
Reflected rays diverge, appearing to come from a virtual focal point behind the mirror. Produces an upright, diminished virtual image with a wide field of view. Used in car wing mirrors.
At a water surface, incident light partly reflects (θᵢ = θᵣ) and partly refracts (transmits) into the water. The refracted ray bends toward the normal because water is optically denser than air (n = 1.33). The refraction angle θₜ is given by Snell's law.
On a rough surface the normal direction varies at each microscopic point, so rays scatter in many directions. The law of reflection (θᵢ = θᵣ) still holds locally at every individual point.