Emissions are substances that are emitted; in an energy sense, these are most often waste products of a process aimed at obtaining useful work. The most common emissions from energy are associated with the generation of electricity and the transportation of people and goods.
When the electron changes levels, it decreases energy and the atom emits photons. The photon is emitted with the electron moving from a higher energy level to a lower energy level. The energy of the photon is the exact energy that is lost by the electron moving to its lower energy level.
A photon can be emitted when an excited electron falls from a higher energy level to a lower level. The energy of the emitted photon will be equal to the energy drop. The energy of an electron and corresponding photon (emitted or absorbed) is expressed in electron- volts (eV's).
Learn more about emission here: