emission

The term emission has a particular meaning in the study of radiative transfer and spectroscopy, meaning the addition of photons to a beam of electromagnetic radiation (e.g., light), such as from changes in atoms, and also from scattering. It is the inverse of absorption, the removal of photons to the beam. The concept is used widely in astrophysics, for explaining and modeling stars, for dealing with the effects of the Earth's atmosphere, for explaining and modeling gas clouds, etc. Each type of emission has an absorption analog (though scattering is both emission and absorption), thus the classification of emission processes also is a classification of absorption processes.

Spontaneous emission is the emission of a photon by an atom not from any immediate interaction, such as the emission of a photon when an atom a lower level of atomic excitation (when an electron moves to a lower orbit). The both the time direction of the emitted photon are random.

Stimulated emission is the emission of a photon by a similarly-excited atom but the emission is specifically triggered by a passing photon of a wavelength that the atom is prepared to emit (i.e., it is excited such that it could emit a photon of this wavelength by spontaneous emission). In this case, the emitted photon has the same characteristics as the photon that stimulated the emission, including its direction. Such stimulated emission is also referred to as negative absorption: on a macro scale, absorption suggests EMR passing through some material is attenuated, but if stimulated emission is occurring, it is possible that more EMR at the wavelength exits than entered. Masers and lasers are based on this effect, by setting up a configuration so that the "doubling" of photons happens repeatedly.

Emission through scattering includes photons entering the beam through Thomson scattering or Compton scattering.

The term emission is also used within astronomy in a more general sense, for the EMR produced by some object, and also for other produced particles (e.g., neutrinos) or waves (gravitational waves).