When light reaches a boundary, part of the wave can return into the original medium. In ray optics this is reflection.

Law of reflection

Angles are measured from the surface normal. The angle of incidence equals the angle of reflection:

$$\theta_i=\theta_r.$$

The incident ray, reflected ray and surface normal lie in the same plane.

Specular and diffuse reflection

A smooth surface can preserve the geometric organization of incoming rays and produce specular reflection, as in a mirror.

A rough surface reflects different small regions in different directions. This diffuse reflection allows an object to be seen from many viewpoints even though the law of reflection still holds locally.

Wavefront interpretation

Reflection can also be understood through wavefronts: different points of an incoming wavefront encounter the boundary at different times, and the reflected wavefront geometry produces the same equality of angles.

Reflection changes the direction of propagation while the wave remains in the original medium, so its frequency and wave speed in that medium remain unchanged.