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The law of reflection states that the angle of incidence equals the angle of reflection, both measured from the normal to the surface. Reflection occurs when waves encounter a boundary and return into the original medium. The law applies to all wave types reflecting from smooth surfaces.
From your study of wave properties, you know that waves carry energy through a medium and interact with boundaries. When a wave reaches the interface between two media — light hitting a mirror, sound hitting a wall, a water ripple reaching a barrier — part of the energy bounces back into the original medium. This is reflection. The law of reflection describes the exact geometry of that bounce with a single, elegant rule.
The critical convention is that angles are measured from the normal — an imaginary line drawn perpendicular to the reflecting surface at the point of contact — not from the surface itself. The angle of incidence (θᵢ) is the angle between the incoming ray and the normal; the angle of reflection (θᵣ) is the angle between the outgoing ray and the same normal, on the other side. The law states: θᵢ = θᵣ. A ray hitting a flat mirror at 30° from the normal leaves at 30° from the normal, in the same plane as the incoming ray and the normal.
Why measure from the normal rather than the surface? Using the normal provides a stable, universal reference. When a surface is tilted, measuring angles from the surface gives a confusing number that depends on the tilt. The normal always provides a perpendicular baseline that cleanly separates incident and reflected rays. This same convention carries forward to Snell's law for refraction, making the geometry of optics internally consistent across reflection and transmission.
A billiard ball bouncing off a cushion obeys the same rule — angle in equals angle out — because the impulse from the surface acts along the normal. Flat mirrors form images that appear to be behind the mirror at the same distance as the object is in front, a direct consequence of the law applied to every ray from the object: each reflected ray diverges as if it originated from a point symmetrically behind the mirror. The spherical mirrors you'll study next apply this same law at every point on a curved surface — but because the normal direction rotates along the curve, different incident rays meet different normals and are redirected to converge (concave) or diverge (convex), producing the focusing and diverging properties that make curved mirrors useful.