Which ray does not aplicable to snels law?

In Snell's Law, which describes the relationship between the angles of incidence and refraction when a wave passes between two different media, the ray that does not apply is the "normal ray." The normal ray is an imaginary line perpendicular to the interface between the two media, serving as a reference for measuring the angles. It's not a physical ray like the incident or refracted rays, which are the actual paths taken by the wave.