The normal to d"S" points in the direction of the "z" axis. Incoming light is confined to a solid angle d"Ω""Σ" and reaches d"S" at an angle "θ""Σ" to its normal. Refracted light is confined to a solid angle d"Ω""S" and leaves d"S" at an angle "θ""S" to its normal. The directions of the incoming and refracted light are contained in a plane making an angle "φ" to the "x" axis, defining these directions in a spherical coordinate system. With these definitions, Snell's law of refraction can be written as