Reflection behaves according to Snell's law which states:

• The incoming ray, the surface normal, and the reflected ray all lie in a common plane.
  
  
• The angle that the reflected ray forms with the surface normal is determined by the angle that the incoming ray forms with the surface normal, and the relative speeds of light of the mediums in which the incident and reflected rays propogate according to the following expression.
  
  (Note: n_l and n_r are the indices of refraction)

Reflection is a very special case of Snell's Law where the incident light's medium and the reflected rays medium is the same. Thus we can simplify the expression to: