The AC current density J in a conductor decreases exponentially from its value at the surface J S according to the depth d from the surface, as follows: J = J S e − − ( 1 + j ) d / δ δ where δ δ is called the skin depth which is defined as the depth below the surface of the conductor at which the current density has fallen to 1/ e (about 0.37) of J S. The imaginary part of the exponent indicates that the phase of the current density is delayed 1 radian for each skin depth of penetration. One full wavelength in the conductor requires 2 π skin depths, at which point the current density is attenuated to e (1.87×10, or −54.6 dB) of its surface value. The wavelength in the conductor is much shorter than the wavelength in vacuum, or equivalently, the phase velocity in a conductor is very much slower than the speed of light in a vacuum. For example, a 1 MHz radio wave has a wavelength in vacuum λ o of about 300 m, whereas in copper, the wavelength is reduced to only about 0.5 mm with a phase velocity of only about 500 m/s. As a consequence of Snell's law and this very tiny phase velocity in a conductor, any wave entering a conductor, even at grazing incidence, refracts essentially in the direction perpendicular to the conductor's surface.