Law

Law of reflection

θ_{i} = θ_{r}

Statement

The angle of incidence equals the angle of reflection, both measured from the normal to the surface.

Symbols and units

θ_{i}, θ_{r}

angle of incidence, reflection

degrees

When it applies

A smooth (specular) reflecting surface.

Don't use when

Assumes a smooth, specular reflecting surface — a rough surface produces diffuse reflection scattered in many directions instead of a single reflected ray.

Derivation

An empirical, directly observed regularity of how light reflects from a smooth surface.

θ_{i} = θ_{r}

Watch out for

Both angles are measured from the normal to the surface, not from the surface itself.