longwavelength

Long wavelength is a relative term used across physics to describe waves whose wavelength is long compared with a characteristic scale of the system under study, such as the size of an object, a feature of a structure, or the wavelength scales being probed. In electromagnetism, the wavelength λ is related to frequency f by λ = c/f in vacuum, so lower frequencies yield longer wavelengths. This places very long wavelengths in the radio and microwave regions, from millimeters to thousands of kilometers, useful for communications, radar, and radio astronomy. In optics, wavelengths longer than visible light fall into the infrared region and beyond, with infrared spanning roughly from 700 nanometers to tens of micrometers.

The long-wavelength regime often implies quasi-static behavior. When the wavelength is much larger than the characteristic dimensions of a system, the electric and magnetic fields vary slowly across the object, and full wave interference is minimized. In such cases, problems can be approached with electrostatic or magnetostatic approximations, and scattering by small objects tends to be weak (the Rayleigh regime).

Applications and considerations vary by domain. Long wavelengths require large antennas for efficient reception or transmission, but they can penetrate through clouds, foliage, and soil, enabling over-the-horizon communication and subsurface sensing. In acoustics, long-wavelength sound has low frequency and interacts differently with obstacles than high-frequency sound. The term is relative and depends on the context and the phenomena being studied.