In vacuum, curl Faraday's law and use the Ampère-Maxwell equation:
Since , the left side is . Hence
The divergence equation requires , and the wave equation requires
For propagation in the positive direction, Faraday's law gives
Thus , , and the propagation direction are mutually perpendicular.
The Poynting vector is
It is the electromagnetic energy flux in the propagation direction; its time average is .

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