Emission

Emission is the process by which a source releases energy, matter, or radiation into its surroundings. It encompasses the emission of light and other electromagnetic radiation, as well as the release of gases, particles, or heat. Emission is the complement of absorption: a body or system can absorb energy or matter and later emit it, or emit it directly without prior absorption.

In physics, electromagnetic emission arises from energy transitions in atoms and molecules. Spontaneous emission occurs without external provocation, while stimulated emission is induced by incident photons and is the principle behind lasers. Thermal emission, described by blackbody radiation, depends on an object's temperature and results in a continuous spectrum determined by that temperature.

Emissions are quantified in various units depending on the type, such as radiant power in watts, photon or particle emission rates, or mass per unit time for gas or pollutant releases. Environmental reporting commonly uses metrics like CO2-equivalent emissions per year to compare the climate impact of different activities.

Environmental and regulatory contexts distinguish natural and anthropogenic emissions. Greenhouse gas emissions from fossil fuel combustion, industrial processes, and agriculture are central to climate policy, and are tracked through inventories, reporting schemes, and emission-control measures aimed at reducing atmospheric concentrations of greenhouse gases.

In astronomy, emission refers to radiation produced by celestial bodies and processes, producing emission spectra with characteristic lines. Such spectra reveal composition, temperature, density, and motion, and are studied across the electromagnetic spectrum from radio to gamma rays.

Overall, emission is a broad concept describing the release of energy or material from a source, observable in many scientific and practical domains.