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.