Long wavelength limit

In electricity and magnetism, the long wavelength limit is the limiting case when the wavelength is much larger than the system size. This corresponds to the quasi-static case, and reduces to electrostatics and magnetostatics.

The physical criterion is that $\lambda >>a$ , where $a$ is the size of the system. The following statements are all equivalent:

$\lambda >>a$
$ka<<1$ , where $k$ is the wave vector
${\frac {\omega }{c}}a<<1$ , where $\omega$ is the angular frequency, $c$ is the propagation speed of waves.

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