The Poynting vector for a wave is a vector whose component in any direction is the irradiance (power per unit area) of that wave on a surface perpendicular to that direction. For a plane sinusoidal wave the Poynting vector is ‍ ⁠ 1 / 2 ⁠ ‍ Re{ E × H }, where E and H are due only to the wave in question, and the asterisk denotes complex conjugation. Inside a lossless dielectric (the usual case), E and H are in phase, and at right angles to each other and to the wave vector k ; so, for s polarization, using the z and xy components of E and H respectively (or for p polarization, using the xy and -z components of E and H), the irradiance in the direction of k is given simply by EH /2 , ‍ which is ‍ E / 2Z in a medium of intrinsic impedance Z = 1/ Y. To compute the irradiance in the direction normal to the interface, as we shall require in the definition of the power transmission coefficient, we could use only the x component (rather than the full xy component) of H or E or, equivalently, simply multiply EH /2 by the proper geometric factor, obtaining (E / 2Z ) cos θ.