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 ${\displaystyle \lambda >>a}$, where ${\displaystyle a}$ is the size of the system. The following statements are all equivalent:

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