The production and release of radiation from a source.Emission is the process by which a source produces and releases radiation, whether electromagnetic waves (like light, X-rays, or radio waves) or particles (like alpha or beta particles). It is a fundamental concept in physics, covering everything from the glow of a hot filament to the radio pulses from a pulsar. Emission is the inverse of absorption: energy is transferred from the source to the surrounding field or medium.
Emission occurs when a system transitions from a higher energy state to a lower one, releasing the energy difference as radiation. This can happen spontaneously, as in the decay of an excited atom, or be stimulated by an incoming photon, as in a laser. The emitted radiation's frequency is determined by the energy gap between the two states, per Planck's relation E = hν. In thermal emission, such as from a hot object, the radiation spectrum depends on temperature, described by blackbody radiation laws.
Emission is harnessed in every light source, from incandescent bulbs to LEDs, and in medical imaging like PET scans, where positron emission is detected. It is also the basis for spectroscopy, which identifies elements by their characteristic emission lines.
The cosmic microwave background radiation, a faint glow filling the universe, is the emission from the hot, dense state of the early universe, redshifted to microwave wavelengths over 13.8 billion years.
