The Law of Reflection:  θ₁ = θ'₁

        The Law of Refraction (Snell's Law):
    n₁ sin θ₁ = n₂ sin θ₂ , or sin θ₁ /c₁ = sin θ₂ /c₂

    _{(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.)}