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 $λ > > a$ , where $a$ is the size of the system. The following statements are all equivalent:

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