Poynting vector

The Poynting vector represents the direction of propagation and rate of energy transfer of an electromagnetic wave at a point in space and time. It is defined as:

\mathbf S = \frac{\mathbf E \times \mathbf B}{\mu_0}

In the diagram below, the Poynting vector points in the $+z$ direction by the right hand rule. Its magnitude varies by its position along the wave and in time.

The time average of the Poynting vector at a point in space is also called the intensity of radiation, and is defined as:

I=\langle|\mathbf S|\rangle

The Poynting vector is also related to power and radiation pressure.