Definition

Electromagnetic wave intensity

(E12.4.1) E12.4 · Energy and Radiation Pressure →

S = \frac{E B}{μ _{0}}, I_{avg} = \frac{E _{m a x} B _{m a x}}{2 μ _{0}}

Statement

The instantaneous energy flux (Poynting vector magnitude) and time-averaged intensity carried by an electromagnetic wave.

Symbols and units

$S$	instantaneous energy flux	W/m²
$I_{avg}$	time-averaged intensity	W/m²
$E, B$	electric, magnetic field magnitude	V/m, T
$μ_{0}$	permeability of free space	4π×10⁻⁷ T·m/A

When it applies

A plane electromagnetic wave, with E and B in phase and perpendicular to each other and to the direction of travel.

Derivation

Analogous to the power carried by a mechanical wave, but built from the electric and magnetic field magnitudes rather than amplitude and mass density.

S = \frac{E B}{μ _{0}}

Watch out for

Radiation carries momentum as well as energy — light striking a surface exerts a measurable pressure, doubled if the surface reflects rather than absorbs, since the reflected wave must also be turned around, requiring twice the momentum transfer.

Connects to

Average power of a wave