Laws of reflection

When a ray of light meets a reflecting surface, such as a mirror, it does not pass through but bounces back. The bounce is not random: it obeys two simple, precise laws, which always refer to the normal, the line perpendicular to the surface at the point where the ray strikes it.

Principle — Laws of reflection

1. The incident ray, the reflected ray, and the normal to the surface at the point of incidence lie in the same plane.
2. The angle of incidence $\theta_i$ equals the angle of reflection $\theta_r$, both measured from the normal: $\theta_i = \theta_r$

The second law is the operative heart of the matter: the light sets off tilted on the other side of the normal, but at the same angle at which it arrived. This simple principle explains why a flat mirror returns such a faithful image.

The incident and reflected rays form equal angles with the normal: $\theta_i = \theta_r$.

Curved mirrors

For a curved mirror (concave or convex) the same law applies, but locally: simply draw the tangent to the mirror at the point of incidence and treat it as a flat mirror. The reflected ray forms the same angle with the tangent as the incident ray.

Collegamenti

Argomenti: Ottica Concetti: Riflessione

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