Reemitting

Reemitting is the process by which a body that has absorbed energy emits radiation again. In physics and related fields, reemission covers several mechanisms by which absorbed energy is returned to the surroundings as electromagnetic radiation, rather than being stored or dissipated as heat alone. Common forms include thermal reemission, luminescent reemission such as fluorescence and phosphorescence, and radiative transfer by intervening media.

Thermal reemission occurs when an object at finite temperature emits a spectrum described by Planck’s law; the intensity and spectrum depend on temperature and emissivity, a measure of how effectively a surface emits relative to a perfect blackbody. Kirchhoff’s law connects emissivity to absorptivity at thermal equilibrium: a material that absorbs well at a given wavelength tends to emit efficiently at that wavelength. Non-thermal reemission, such as fluorescence, occurs when absorbed photons promote electrons to excited states, which then decay and emit photons of typically lower energy with characteristic lifetimes.

In astronomy and planetary science, reemission explains why dust grains and atmospheres glow in infrared after absorbing visible or ultraviolet light, contributing to observed spectral energy distributions. In climate science, reemission by greenhouse gases returns infrared radiation to space and toward the surface, influencing the planetary energy balance. In materials science, luminescent reemission is exploited in phosphors, LEDs, and optical sensors. Reemission phenomena are central to interpreting spectra and to the design of radiative technologies.

See also: fluorescence, phosphorescence, blackbody radiation, radiative transfer, greenhouse effect, emissivity, Kirchhoff’s law.