Editing Laser (section)

=== Stimulated emission ===
{{Main|Stimulated emission}}

[[File:Laser, quantum principle.ogv|thumb|upright=1.5|Animation explaining stimulated emission and the laser principle]]

In the [[Classical electromagnetism|classical view]], the energy of an electron orbiting an atomic nucleus is larger for orbits further from the [[atomic nucleus|nucleus]] of an [[atom]]. However, quantum mechanical effects force electrons to take on discrete positions in [[Atomic orbital|orbitals]]. Thus, electrons are found in specific energy levels of an atom, two of which are shown below:

[[File:Stimulated Emission.svg|frameless|center|upright=2]]

An electron in an atom can absorb energy from light ([[photon]]s) or heat ([[phonon]]s) only if there is a transition between energy levels that match the energy carried by the photon or phonon. For light, this means that any given transition will only [[Absorption (electromagnetic radiation)|absorb]] one particular [[wavelength]] of light. Photons with the correct wavelength can cause an electron to jump from the lower to the higher energy level. The photon is consumed in this process.

When an electron is [[excited state|excited]] from one state to that at a higher energy level with energy difference ΔE, it will not stay that way forever. Eventually, a photon will be spontaneously created from the vacuum having energy ΔE. Conserving energy, the electron transitions to a lower energy level that is not occupied, with transitions to different levels having different time constants. This process is called [[spontaneous emission]]. Spontaneous emission is a quantum-mechanical effect and a direct physical manifestation of the Heisenberg [[uncertainty principle]]. The emitted photon has a random direction, but its wavelength matches the absorption wavelength of the transition. This is the mechanism of [[fluorescence]] and [[thermal emission]].

A photon with the correct wavelength to be absorbed by a transition can also cause an electron to drop from the higher to the lower level, emitting a new photon. The emitted photon exactly matches the original photon in wavelength, phase, and direction. This process is called stimulated emission.