On the other hand, calculation of the power transmission coefficient T is less straightforward, since the light travels in different directions in the two media. What's more, the wave impedances in the two media differ; power (irradiance) is given by the square of the electric field amplitude divided by the characteristic impedance of the medium (or by the square of the magnetic field multiplied by the characteristic impedance). This results in: T = n 2 cos ⁡ ⁡ θ θ t n 1 cos ⁡ ⁡ θ θ i | t | 2 using the above definition of t. The introduced factor of ⁠ n 2 / n 1 ⁠ is the reciprocal of the ratio of the media's wave impedances. The cos(θ) factors adjust the waves' powers so they are reckoned in the direction normal to the interface, for both the incident and transmitted waves, so that full power transmission corresponds to T = 1.