= Electromagnetic wave
{wiki}
An <electromagnetic wave> is a propagating disturbance of the <electric field> and <magnetic field> obeying the <Maxwell equations>. In source-free vacuum its speed is
$$
c=\frac1{\sqrt{\mu_0\epsilon_0}}\simeq2.998\times10^8\,\mathrm{m/s},
$$
the <speed of light>. An <electromagnetic wave> can travel through vacuum: a conducting medium is unnecessary. A single traveling <plane electromagnetic wave> has mutually perpendicular <electric field>, <magnetic field>, and propagation direction. This statement applies to that wave; arbitrary <near and far field> configurations need not share these directions or the relation $|\mathbf B|=|\mathbf E|/c$.
The visualization compares perpendicular components on a common scale: $c\mathbf B$ has the same units as $\mathbf E$. The curves are field values along space, rather than paths traced by particles. Their common phase moves toward $+z$, while the <Poynting vector> records the associated <energy flux>.
\Image[/electromagnetic-plane-wave.png]
{title=Vacuum plane wave with electric field along x, magnetic field along y, and energy flux along positive z}
The <electromagnetic wave equation> gives a propagation law, while the full <Maxwell equations> additionally enforce the field directions and relative amplitudes. This is why solving an independent scalar <wave equation> for each component is insufficient to construct an <electromagnetic wave>. https://www.feynmanlectures.caltech.edu/II_20.html[Feynman lectures, II.20: solutions in free space]{ref}.
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