Reradiation

Reradiation is the emission of electromagnetic radiation by a material after it has absorbed energy from another source. It is a form of radiative relaxation and can occur through several mechanisms, including thermal emission, fluorescence, and phosphorescence. In thermal reradiation, absorbed energy raises the object's temperature, and the object emits a spectrum described by blackbody or graybody radiation, governed by its temperature and emissivity. In fluorescence and phosphorescence, energy is temporarily stored in excited electronic states and released as photons, typically at characteristic wavelengths, with fluorescence occurring rapidly and phosphorescence on longer timescales.

The spectral distribution of reradiated light depends on the mechanism and material properties. Thermal reradiation follows Planck's law and broadens with temperature, producing continuous spectra that shift toward shorter wavelengths as temperature increases. Fluorescence yields discrete emission lines or bands related to electronic transitions, while phosphorescence involves delayed emission after the excitation source is removed.

In planetary and astrophysical contexts, reradiation strongly influences energy balance. Dust and gas that absorb stellar radiation reradiate primarily in the infrared, shaping thermal emission measurements of galaxies and the interstellar medium. In planetary atmospheres, greenhouse gases absorb infrared radiation and reradiate it in all directions, including back toward the surface, contributing to warming.

Kirchhoff's law relates reradiation to absorption: for a body in thermal equilibrium, emissivity at a given wavelength equals absorptivity. Thus a material's reradiative properties reflect its absorptive behavior, with real materials exhibiting spectral features and temperature-dependent deviations from ideal blackbody behavior.

Applications of studying reradiation include infrared astronomy, climate modeling, and materials characterization, where emissivity, radiance, and radiative transfer are used to describe and predict observed emissions.