The Law of Reflection:  θ = r

      The Law of Refraction:  n₁ sin θ = n₂ sin θ'
  or, sin θ /c₁ = sin θ' /c₂ or, n sin θ = constant.
    _{(In the slower medium, the light ray is closer to the direction of
  the normal.  If medium #2 is opaque, there is no refracted ray.)}