localfield

Local field refers to the effective electromagnetic field that acts on a specific particle, molecule, or spin inside a material. This field is the combination of the externally applied field and the contributions from the surrounding charges, dipoles, or magnetic moments. The local field generally differs from the macroscopic field that is measured or applied to the bulk sample, because it includes microscopic interactions with the immediate environment.

In dielectrics and polarizable media, the local field at a molecule can be significantly larger or oriented differently than the external field due to polarization of neighboring molecules. A common model is the Lorentz field, where the local electric field is written as E_loc = E_ext + P/(3ε0), with P the macroscopic polarization and ε0 the vacuum permittivity. This correction underpins the Lorentz-Lorenz relation, which connects refractive index to molecular polarizability and number density.

Local-field concepts also apply in magnetism and other condensed-matter contexts. The local magnetic field at a lattice site includes the applied field and the contributions from nearby magnetic moments; the exact form depends on lattice geometry and interaction details.

Calculation methods range from continuum approaches like the Lorentz cavity model to discrete-dipole models and Onsager reaction-field methods for polar liquids. Local-field effects influence linear and nonlinear optical responses, dielectric spectroscopy, and spectroscopic shifts, making them essential for interpreting material behavior at the microscopic level.

The term originated in early 20th-century work by Lorentz and colleagues and remains a central idea in understanding how microscopic structure shapes macroscopic electromagnetic properties.